Graffiti is vandalism plain and simple. If one wants to express their feelings with cans of spray paint then get the owner's permission or find a sponsor. If one is desperate for a canvas just turn the nozzle around and spray the paint on ones own corneas.
Could not agree more. An "art" course at a local university here actually had a course in making "Graffiti Art". When I called and complained that this was encouraging vandalism, they told me that was nonsense.