It is not the act of attacking a place that is the problem. It is how she did it.
The Alliance have treated them as monsters because that's all the Forsaken have ever been and done to them and for good reason. The first time they encountered a splinter of the Alliance they betrayed and slaughtered them all, following this she appoints a freaking Dreadlord as her second in command. Tell me why should be trusted if that is what they did as their first action as a people.
Why should the Alliance treat them any differently to what they've shown themselves to be?