Scrap the bomb, the other leaders were against marching on Theramore regardless. Theramore had long been the only bastion of truce between Horde and Alliance, led by a long time ally and supporter of Thrall, and thus one of the few friendly voices supporting the Horde as being non-monsters among the Alliance. Vol'jin and Baine knew this and were against the attack from the start. Hell, Baine warned the Alliance ahead of time to try and save them if possible. They were against attacking Theramore, period.
And they were even more pissed when they realized he sacrificed Horde soldiers just to put on a show before dropping the bomb. The bomb may not have killed them, but the Horde was being killed and defeated by the Alliance. If only Garrosh knew of the plan to drop the mana bomb, you can be assured the Horde soldiers were fighting with all their might to claim the city. They were sent in to a fight without need, a fight Garrosh had no intention of concluding. That's why Vol'jin and Baine felt it was dishonorable and that Garrosh betrayed his own soldiers.
My question is more along these lines:Is it? According to whose laws?
You've accepted the dragon enslaving Dragonmaw Clan and now you've slain members of the Blue Dragonflight and stolen the Focusing Iris. HOW does the Horde not have the Dragonflights turning against them under Garrosh at this point? There's no real rational explanation short of plot armor.