Garrosh didn't send troops to Thunder Bluff not because he wasn't ready to help the tauren - the Horde race he had in almost as high esteem as orcs given that they were the only ones besides orcs allowed to live in the Valley of Strength, but because Baine preferred to ask Jaina, as is always the case.
@Le Conceptuel
It's simpler than that. The druid school was useless as a military target and was entirely civilian. Theramore was a military target and anyone who stayed behind or was in range was pretty much fair game and were going to, by virtue of his strategy, likely going to have the chance to evacuate anyway. Even in Mists Garrosh condemns the same things he condemned in Stonetalon when he tells Ishi that there's a difference between killing and murder once Ishi says he wants to chop Alliance kids in their sleep.