You put the counterpoint yourself: the Alliance didn't do half of what the Horde did in terms of aggression. The assault on the Barrens was basically the only Alliance assault upon Horde territory in Cataclysm. The Alliance doesn't have any reason to know Sylvanas is a loose cannon besides what some Worgen who listened in at the Cathedral in Gilneas would have told them. Sylvanas isn't the only reason the Alliance is mad at Garrosh, he's ordered or was blamed for plenty more. Also, the upcoming razing of Theramore seems to be his own idea.
And let's all keep in mind that Garrosh will be a boss in the final raid of Mists of Pandaria. We've got at least two large patches of content to show us what else he's got up his metaphorical sleeves. Theramore is the first step and we'll have an entirely new continent to ruin. He'll be up to a lot more by the time we come take his loot.