It's only unjustified because Garrosh decided to torture and publicly slaughter the innocents of Theramore post-bomb. Oh, and he also bombed it mostly without warning. Only Baine had the warn Jaina about it, and he did it outside of Garrosh's command.
The attack itself was mostly justified, because of the situation at the Barrens. Intention or not, what happened there is how it happened. The attack was used as a counter to push the Alliance out of Kalimdor because of their shit at the Barrens.