Presumably Banksy and associates would have been arrested too if they had been caught. This whole thing relies on doing it in a way that people don't question it while it's happening.

Yeah, and that is precisely the point.

This contradiction at the heart of it does a lot of work and is a very valuable part of the art. This contradiction has led me to think a lot about rules and their role in society and to what extent pure strict rules based societies are a worthwhile goal and on the other hand what it means of we make exceptions.