This is patently absurd. He's my apprentice, and he'll soon be elevated to the single most glorious position in the world, that of Hermetic Magus. I think giving him that opportunity more than entitles me to do whatever I please with him in the interim; it's not as if he'll pay me back later for good treatment.
Joking aside, 'abusing' your apprentice usually just means you aren't teaching them for one season a year or are keeping them apprenticed past 15 years. You can literally murder your apprentice at any time and not face any censure from Hermetic law. Apprentices are basically slaves. But training them properly is required, because the Hermetic definition of an apprentice is pretty much somebody you train one season a year. Fail to do that and your apprentice is 'legally' free to seek out someone else to teach him and they can claim the apprentice on the spot.
Ah, you refer to the legal form of abusing your apprentice, in terms of exploiting them for lab assistance without teaching them (properly). Then there's no disagreement to be had here.
(As a largely unimportant side note, it's interesting that there are actually "good" reasons for the fact that murdering your apprentice prior to them becoming magi is considered more acceptable than not teaching them the required amount... Namely, it reduces hoarding of Gifted children for lab benefits, since even an apprentice you intend to murder later saps a season every year, so most would consider more than one to be cancelling out their own benefits. One dead child is a small price to pay to assure that all the Gifted children the murderer would have otherwise hoarded go to masters that will actually make them magi.)
Snatching for liberation and justice goes beyond simply addressing issues of correct training. There's many scenarios where a Bonisagus might snatch an apprentice for the 'greater good' - and equally as many where the former master will look to some knife-sharpening in response.
A magus who is heavily suspected of diabolism takes an apprentice. Rumour has it he has a ritual to grant him great power by sacrificing a Gifted child to his dark masters. Snatch?
The nice thing here is some people will say 'yes' and some will say 'no.' But in either case, it's legal so there's no Tribunal case, and there's no waiting for the next full moon to disrupt the (supposed) ritual.
This is one of many scenarios where a Bonisagus magus might snatch to 'rescue' an apprentice. Sometimes they'll get it wrong, and the affronted magus will be angry.
But hey, if you have a terrible Hermetic reputation, or a family history of diabolism, then you had it coming. Especially the PCs who picked that as a flaw. 
Then there the magi who kill or cripple apprentices doing experiments. In Houses of Hermes - True Lineages page 54 there was the maga who claimed and killed 8 apprentices. The Quaesitor insisted she be acquitted so the next time she claimed an apprentice she got 7 declarations of Wizard War at the full moon.
Given how small the Order as a whole is, the size of a House can vary enormously based on random flukes.
Someone from House A trains 5 apprentices, and tells them all that they have a duty to train more people and pass on their knowledge. Someone influential in House B very convincingly talks to their House-mates about how miserable raising an apprentice is. Someone in House C makes a serious effort to gather as many Gifted kids as possible for their House. Someone in House D enrages the Aesir, getting themself and ten of their closest friends killed.
Fifty years later, House A and House C are noticeably larger than House B and House D.
Let me repeat something that has already been said (and hopefully add something new).
Bonisagus magi can take the apprentices of other magi, so they have an easier time obtaining an apprentice ... but they also have extremely high standards and tend to put extra effort into teaching (see e.g. fostering), so it sort of balances out in terms of how many apprentices each magus "produces". In this sense, I would think few magi from other houses would begrudge giving House Bonisagus one apprentice, including Merceres: it is an honour and a potential networking opportunity, very much like a professor from one of today's mid-ranking academic institutions seeing one of his undergraduate students leave for graduate studies at a top-ranking school (and in any case, house Bonisagus does pay back the Order with Free [strike]Software[/strike] Magic Advances).
Also, keep in mind that House Bonisagus is a True Lineage, and True Lineages are at an inherent disadvantage in terms of numbers, because there will be a constant trickle of senior magi leaving the House for a number of reasons, but the reverse stream is blocked. Note that of the other True Lineages, both the Tremere and the Mercere explicitly put a lot of effort into trying to keep their numbers up, and the Guernicus ... well ... the Guernicus should probably be the smallest house in the Order, and in my sagas there are always several conspiracy theories of why this is not the case
Bonisagus isn't that small - The Mercere are smallest, and three of the mystery cults are smaller. The societates are meant to be big, because they recruit widely, have incorporated a range of styles into each house, and have all encouraged the join part of join or die at some point. So really, the question is why is Criamon so successful for a Mystery House, and why are Guernicus and Tremere bigger? HoH:TL says there are 81 bonisagus, 98 Guernicus, 92 Tremere. So actually they are only a litle smaller than the Tremere, who dominate a Tribunal and may or may not be planning to take over the world. Guernicus probably need the numbers as 98 Guernicus divided by 13 tribunals is only seven and a bit lawkeepers per tribunal, and one or more quaesitor per house.
As for Criamon's success? Make your own theory. I reckon The Enigma helps Criamon find suitably Gifted and inclined potential apprentices.
I'll make my own theory, sure. First, House Criamon is full of pacifists who don't start trouble and therefore don't get killed in marches or wizard wars. Second, they don't seem to be in the business of pushing magic theory, so they explode a lot less often. They hang out with other Criamon in mutual support groups that actually help each other. They tend to avoid messing with the mundanes at all.
In other words, Criamon magi suffer a much lower attrition rate than every other house (even accounting for more frequent twilights, which Criamon magi weather fairly well).
Hi,
Having more Twilights even boosts population, because Criamon in Twilight do not age, are impervious to harm and pick up living right where they left off. Magi who enter Twilight often might live for many, many objective years, though fewer subjective years. Criamon on Sabbatical (7 year Twilight 
) do count toward the population of the house, as do those 5-station Criamon elders waiting upon eternity.
Anyway,
Ken
In my mind, there is another reason why the First House has lower numbers than the majority of the order...they are basically in a long Winter.
Think about it, very few new discoveries, a rather conservative leadership, a (possibly illegal) disinclination to share their knowledge base, a rather significant amount of arrogance, discounting new ideas...if we were talking about a covenant rather than a House, it would certainly be Winter.
As for snatching...I can easily imagine that several of those apprentices go to their old master's House upon completing their Gauntlet.
Oh, I had forgotten those guys.
But would another of the True Lineage accept a new member? Normally not but a magus who started out as Tremere, Guernicus, or Mercere but was Boni-snatched at some point during apprenticeship and finished as a Bonisagus?
And would it matter how long the apprentice was trained in the original house? Potentially more secrets known which the house wishes to keep and hence take back into the fold?
Would the 3 houses have much different opinions about this? If the magus in question was a blood relative of a magus Mercere?
Mystery Cults may be secretive but are known to accept new members after their Gauntlet. But how do they feel if someone left? Once opened to their Mystery the apprentice is forever changed so they are always part of it .
I'd say it varies based on House, though True Lineages & Mysteries have different outlooks.
I may be giving the Order too much credit.
Which brings up another question- If an apprentice is Bonisnatched who has already been initiated into a mystery (either mystery lineage or mystery cult) how does that interact with the requirement that a Bonisagus must make their research available for sharing? If you have been initiated Verdituous, then become Bonisagus, and must share your lab texts for enchantments...
Their oath actually says "I will further the knowledge of the Order and share all that I find in my search for wisdom and power."
Naturally, this is open to interpretation - the main part being "I find" can be interpreted as 'what I research' NOT 'any information I come across' - otherwise a Bonisagus could not keep ANY secret about anything without breaking his Hermetic Oath. Also, the word 'share' is already being interpreted as 'submitted to the house Bonisagus for codification', and the 'Order' can be interpreted as fulfilled so long as ANY ONE other member of the Order has knowledge of it (in this case, they Mystery Cult has the knowledge, so the Order has the knowledge, just not every member of the order). A Bonisagus approached for mystery cult secrets can also just point the person in question towards the mystery cult - the knowledge is already out there, so he is not actually concealing anything. A magus cannot force a Bonisagus to surrender to him a clear text of a well known spell (Aegis of the Hearth ReVi20) upon demand, that spell is already in distribution. There are rulings in place to prevent magi from wasting a Bonisagus' time in that fashion. Mystery cult virtues are in distribution in the Order as well, just in a more controlled, limited fashion (and they do actually recruit).
Many of these interpretations might not stand in tribunal, but the mystery houses have more than enough pull to insure it never comes up, and the mystery cults have certamen, wizard's war and other means to keep their secrets. The most likely scenario I see happening if this can't be stopped from going to tribunal is that the Prima of Bonisagus declares the magi in question Orbus and he joins another house, so he does not have to share anything. It's better than him getting killed by the mystery cult or being marched for violating his oath.
I like this explanation, particularly because I view the Order itself in 1220 as starting to fall into Winter, and part of the reason is definitely that Durenmar and Bonisagus should have already fallen into it - the fact that they haven't is because nobody's come up with a replacement, and even a calcified Durenmar is still the most valuable resource around.