If he is being called a muralist, perhaps he was working legally.
I don't know if you are aware that some cities condone artwork on certain structures. In Chicago these are often specifically permitted with certain neighborhood themes on the walls of railroad overpasses. Also there is an industrial area with concrete-walled railroad embankments for over half a mile and each panel of the concrete - maybe 15 feet square - is another mural. Some are mundane, some political, the talent level varies but there are many interesting works and some are phenomenal. That's what I thought of when I read 'muralist'.
