Why does House Tremere forbid Dampir from joining the Order of Hermes? And why do the other houses let them do this? I would think that House Merinita at the least would object to the fact that a group of gifted people can't enter the order simply because they have faerie blood that House Tremere doesn't aprove of.
If it does not fit your vision of mythic europe, just ignore it. The background is full of stuff thasst you should ignore to have a coherent game world, so floating wehat the supplement says is no biggie. the only one that has a definitive answer here would be Timothy Fergusson (the author) but it might be a heritage of "vampires not welcome in Ars Magica" statement after White Wolf (when they controlled the game) made the game shift too crazed out tribunals full of demons, evil people, werewolves and vampires.
The houses maintain the right to accept and kick put whichever members they choose. No primus or house organization is going to take another house to task and thereby imply that they shouldn't be allowed to choose their own members.
It' because there was a canonical note in a porevious edition that the House had suffered a mild vampirei nfestation, then killed them all off. Dhampirdom fit the bill on that.
In character, why take an apprentice who, at death, unless careful precautions are taken, will become demonically possessed serial killer or a faerie powered ghostly rapist while still being able to cast spells and may be able to train outsiders to use the Parma Magica? Faeries who know the Parma Magica are bad news all around. There comes a point where you go "Hey, killing that last Tremere vampire acrhmagus was just too damn difficult. Let's not do that again. Ever."
So tempted to start yelling lich.
Childish I know.