I agree 100%.
But anyone should - your question begs its own answer, because the question assumes the action is "completely legal", end of statement.
There are examples in the Peripheral Code where a mage commits legal action X, which directly causes questionable action Y as fallout. The Tribunals rule that action Y is still illegal - claiming that "action X is legal" is irrelevant.
Two examples that I've found:
1151 - Durenmar has a legal right to vis sources, but over-harvesting causes enmity of the Fae - guilty of the latter, regardless of the former.
1194 - William FIreheart legally kills a mundane, and in so doing accidentally damages a lab. The killing was legal, the damage not. Guilty of the latter, regardless of the former.
The relevant pattern demonstrates that altho' the claiming is legal, it still cannot then break a different part of The Code. And in no counter-example that I've seen is there any suggestion that the plaintiff proved that the apprentice was worth vis, nor asked for compensation based on that, nor cared for such - ONLY that they should have/keep claim to the child.
If the issue is not raised to the Tribunal (and the Tribunal does not mention it in their findings), then the Tribunal is not ruling on that issue, but a different one.
Lastly, these 2 entries...
1188 Provencal: Magus Teslil of Jerbiton finds a Gifted girl and, having his own apprentice, sells her to Magus Gentric of Jerbiton.
...
1194 Provencal Tribunal of 1333:...Magus Teslil claims that the former apprentice of Magus Gentric, died 1331, should be granted him, as Gentric had been of his House and he had found the girl to begin with. The Tribunal rules that Teslil, having sold the girl, had given up all claim . (emphasis added)
I submit that you cannot "give up all claim" unless you have "some claim" to begin with. If the mage selling the apprentice "had no claim", the Tribunal would have simply said as much and in so many words.
These are by no means "proof", but they do support my position to some degree. However, if you feel that other rulings re Claiming also rule silently on my position (without ever specifically saying anything about it), that's fine.
This thread has still, quite inarguably, given me a definitive answer on that point at least (and among others) - that this matter is not clear, and if it were to go to Tribunal it could be completely unpredictable. 