Yeah... I can call you a traitor for for even constructively criticizing a military operation, or for switching lords within the realm, or for doing my every bidding, can't I?
Why wouldn't I be able to call someone a traitor for seceding from the realm?
The point was, you can call them a traitor but game mechanics prevent you from branding them as a traitor. Even if you capture them, unless they were engaged in black market or infiltrator activities you still can't ban them.
