Can you guarantee that? Maybe you have your liege lord's permission. Maybe the terms of your oath to your lord have been fulfilled, and the oath is no longer binding. Autobans fail to account for all kinds of things like that.
But if you're going to claim that any allegiance change to another realm is breaking your oath and deserving of a ban, then you have to extend that to all realm allegiance changes.
If you're saying "changing your oath within the realm should get a ban," that doesn't make sense. There is currently no region-ban or duchy-ban, and the oath
to the realm has not yet been broken.
But if you're saying "changing to another realm should always get a ban", I'm OK with that.
The game
cannot figure out what kinds of RP oaths people make with each other.
The game
cannot know whether a secession is friendly.
The game
cannot, in general, know what the players are thinking, and therefore must make certain assumptions.
The players, on the other hand, can both know what they are thinking and take actions to compensate for inflexible game assumptions.
One of those actions is lifting autobans.
Thus, I submit that autobanning in
every case is significantly better than autobanning in
no cases.
I would, however,
prefer autobans only if the realm being joined is at neutral or lower relations—possibly peaceful relations too, not sure about that.