This thing called "rules of land ownership". It not belonging to anyone made it up for grabs.
That's like, your opinion. Not supported by anything. The attacked party may overreact as they please. Especially in a world without really established rules of war. Also, your comparison doesn't really work. South Korea isn't in the same kind of military alliance with US as the Alliance members. And given how the operation in Silithus is multi-racial, it'd be an operation of US AND South Korea. So yes, Russia may invade the shit out of South Korea if they participate in an attack against them.
Or perhaps they will not. Until that actually happens the prospect of a retcon is meaningless. You could argue the same thing about everything. Whoopty doo, Illidan was never freed from the Barrows Den by Tyrande because it can be retconned. And the Alliance quest doesn't actually require the player to kill the miners. Only sabotage the camp. You know, spy stuff. The miners only attack when the player's spy activities are detected. As is reflected in the Horde quests.
And not just mining camps. Both factions set military camps all over Northrend. Something something by what right!!1!