Seems like people are sensitive to this issue and are getting defensive. Art or not, Putting paint on something that doesn't belong to you, without consent is fucked. Deserves a punch in the face and a bill for the clean up.
How would you feel if someone painted all over your favorite possessions? Like your house, car, skis, whatever you like best. Imagine some asshole painted some random picture on your shit. Sucks, makes you want to punch them in the face and make them buy you new shit.
Of course, art done right is cool.