I think some of you are jumping down the OP's throat.
I don't immediately get the impression that he's against donations or even donation rewards, I think he's specifically targeting the obtuse-ness of horde being able to ride mounts that they otherwise never would have been able to ride in a legitimate vanilla setting.
No, it doesn't affect anyones performance, and no, it doesn't hurt anyone in any digital way what-so-ever, but it can hurt the immersive feeling that people come to these servers to experience. Everything plays in to that, not just game balance or an un-tinkered progression style meant to mimi retail vanilla. This is an extreme example but let's say they replaced every single in-game NPC with a character model of an Orc, just for giggles. Nothing would change. It would affect nobody in terms of performance or game balance at all, but would it not feel a bit...uhm...weird?
I think that's what the OP is aiming at, and to be quite honest I agree, I think it's slightly if a little bit "erm" when you see it, it does remove a bit of the immersive, nostalgic punch that generally Kronos is otherwise superior at providing.
To make matters a bit more pickled, it wasn't originally intended to be like this. It was all due to a bork in the system that failed to prevent people from purchasing opposite faction-only mounts and being able to use them.
Honestly I think it's a little unintuitive to take the "Oh well" route with this situation. They should remove all mounts purchased by opposite faction players and refund their stars, allowing them to purchase appropriate donation mounts. There shouldn't be any complaints about "But the only reason I donated was to get this mount!" because it wasn't even intended to be that way in the first place, they knew that when they donated, and anybody who did after the first person exploited is just a collateral failing of the system.
I don't think anyone at all would have an issue with them taking such steps to make sure the realm provides a more nostalgic impression, and less of a break in that feeling because of races riding on impossible-back-then mounts. This is my suggestion anyway.