I understand what you mean, but the way the game uses the de jure system appears not to be strictly what the term means in the historical sense. It's a combination of that and a gameplay mechanic. I'm not sure if the game even could handle having half the map not follow any de jure rules.
The game as it currently is couldn't handle it.
However, this thread is not for the game as it currently is. It's a gathering point for our wishes for the next version - and one of my biggest ones is to more truthfully represent the different forms of ruling systems in place in this time period.