Personaly I have no problem with bribes the way they are...

I rationalise it like this..

The rebel faction and the general represent the guys in control of the Independent state..

When you bribe them you are buying That general and his soldiers loyalty, it's basically a sponsored invasion... Once they become member of your army you can treat them as any other subject.

Ideally there should be a whole raft of nobles, titles and relationships, but that only covered in a very abstract way in the game and the present system seems ok...