Really? Wow, never knew that.
Personally I do not limit classes by race, and I also use the sneaky elf rule as SD is a prime requisite for Stalk & Hide.
I feel if the player wants to be a bad thief, go ahead and take the War Troll, but remember Everyone Hates you, and you are not very good at sneaking, or Hiding. :-) Then again, I try to prevent Power Gaming as well, so give me a good reason why you are a war troll living in the capital city of the human empire. Back storey is something I insist the players develop and work with me on.