Just to clarify, I think it's fine if a protectorate decides it doesn't want to be a protectorate anymore. Of course, they should get spanked in the diplomacy system for backstabbing.

What I think should be impossible is for a protectorate to declare war on some 3rd party and then expect their big brother to finish what they started. Protectorates should not be dragging their protector into wars. If anything, it's the other way around. That's the tail wagging the dog, which is dumb.