i work along a rail siding on many days and yeah, the gamut of skills and intent behind the works and their authors are as wide as a train is long..mind you practically none with nefarious implications as they just roll on by.
I think there should be a rule. If it's just some gang banger scrawling the name of his little band of thugs, they can paint over it, but if it's something beautiful like that, they should leave it up to the owner of the business whether to leave it.