So no rebellion is ever principled? Do they lose their moral legitimacy as soon as someone fires a shot?
You use the word "rebellion" in a country that specifically states it is everyone's right to form a government that suits them. A country that was specifically founded on the idea that you had a right to leave a larger country of which you no longer wished to be a part.
I would say that in a country that has stated such a premise in it's founding document, any effort to stop a breakaway is unprincipled. Far from being a rebellion, it is the ultimate statement in support of the very foundation of this nation. The right of association also asserts a matching right of disassociation.
Those who deny this right are the actual rebels.