Which isn't too surprising -- Hunters have a relationship to wild animals, not trained ones, so the fact that you can't steal bears from the trolls falls in line with Hunter lore (although, if you wanted to make the case that the bears were actually pre-trained and that it would make them easier to train, you probably could, even if Blizzard didn't listen). And it's not like there aren't other bears to get in the game. Blizzard should definitely have some fun pets to get like this (as long as there's no huge gameplay bonus to be gained), but apparently Zul'Aman bears aren't one of them.
Of course, you can always go get your bear mount -- we've got a guide that tells you exactly how to do it. And hurry -- even those won't be available forever.