I actually fail to see how this is relevant in most cases. Several times in history forts (and provinces) have been annexed without being occupied by an attacker. But my main problem is how it is extremely unrealistic in most cases: I am Austria and I am occupying all of Papal States EXCEPT Avignon. My warscore is 99%. I still can't release Avignon as independent state (-1000 due to un-occupied fort). I could understand potentially a slight negative modifier, but -1000 is surely absurd. I mean, if a nation is utterly beated and defeated except a remote fortress, with its capital and all core lands under occupation, how can they deny until the end of time the transfer of an unoccupied fortress?