Imperialist paradigm? Can you elaborate on that please? Why would it prevent them from eliminating a hostile force within their lands?

Sure. See here. 

The paradigm is that the Andals sailed to the Vale and seized the land by force, violently converted the remaining population to a foreign religion, and then drove the resisters into the high mountain valleys, as the “steepest places have been at all times the asylum of liberty.”

There’s not much of a basis for peace. The Andals aren’t going to give up their lands, the Mountain Clans aren’t about to stop raiding their lands. So the status quo is vile, and “eliminating a hostile force within their lands” even more so.