Who "owns" graffiti? The property owner of the wall it was sprayed on?
Probably, aye. Seems to work that way. Doesn't stop councils cheerfully erasing it from time to time though, viz. the case with another Banksy piece.
Someone should totally do a remix of Jay-Z's Picasso Baby. s/Picasso/Banksy
I'm not sure we can talk about an "owner" at graffities. They sould be considered as "liberated from any owner" or "owned by community". But when people focused on money (I'm not eating "the charity trick"), ethics can't do anything important.