Did you read the linked article? [1] The only thing that matters is if the property owner likes it. If the property owner likes it, it stays up, if they don't, they file a vandalism case with the city.

Banksy is a vandal, as I'm sure anyone would admit, himself included. I am equally sure that there have been times when property owners haven't liked his work and have tried to report him for vandalism. But given he doesn't make a habit of posting videos of his actions online and bragging about them, he doesn't get caught/attributed.

https://www.tmz.com/2014/04/02/banksy-david-william-noll-ric...