if Banksy's painting without the property owner's permission, it's not an act of rebellion or whatever, it's graffiti.
It's weird that you seem to think that, in order to be an act of rebellion, something must be legal. You also imply that a thing cannot be two things.
When John Wilkes Booth assassinated Abraham Lincoln, he committed a crime, an act of rebellion, and a murder.
Personally, if Banksy tagged my house, I would be so fucking happy. I could sell my house for a shitload if it had an original Banksy on the side. That would be like getting mad if you went on vacation and someone broke into your home, didn't steal any of your stuff, but completely remodeled everything so it was way better than before. Sure, they committed a crime, but it would be weird to complain about it.