We put them in jail, too, depending on the building they deface and/or the attitude of the private property owner. Street artists also get commissioned to make it look like the establishment has been "chosen" as a canvass to pump it's cool factor. So the police tend to act only on a complaint or where a public building is involved. Every once in a while the police go on this kick where they believe street art and graffiti create an atmosphere for criminality so they actively remove the art from the walls. It's called "broken window" policing. If gangs and others see a street in disarray then they move in, goes the theory.