This reminds me of the ridiculous "revolts" in MTW, and the cheats involved : a good way to kill a AI opponent is to create revolts in his former provinces ... and then buy the revoltees to add to your army !
I think that the problem in Dom2 is basically the same : the "unrest" is not relative to the player creating it, but an "overall" turmoil state, allowing one player to "buy" it for its opponent...
Maybe when one province is conquered unrest should be reset, or at least seriously lowered, so the new owner doesn't get all the mess the former did
