They put up with it because it ultimately served Azeroth.
Though to be fair we haven't even seen those parties react. If Alexstraza shows up next expansion and doesn't say anything it's one thing, but we haven't seen that yet.
Dude, i seriously question what you're thinking if you actually believe Anduin is a warmonger... He has a good reason to go after Undercity. Not everybody who goes to war is a warmonger.Not sure how it's trolling to state that Anduin, who's spent his life until now arguing for peace, launches an assault on the Undercity and fights on the frontlines himself.
Jeez, Legion really did a number on the perception of these factions. The Horde and Alliance are enemies. Always have been.