This photograph by Daishin Stephenson was taken on the canal bank in Phoenix.
The common argument that graffiti is vandalism strikes me as being about class, not art. While much graffiti is ugly and artless, so is much sanctioned (and paid for) “public art.” What causes graffiti to be called vandalism is its being painted, without permission, on surfaces that the artist does not own.
Look at the sanctioned art in public spaces. Who gave permission for it? Not me, and probably not you. Much of what I see is so mediocre or dishonest that I would not have permitted it had I been asked, so it is just as much vandalism as the work of a tagger. If public spaces are truly public, then they are ours to paint and write on, or they are no one’s.
There are some good documentaries about graffiti - I recommend Infamy, Next and Bomb It.