There are many people who believe that street art, more commonly known as graffiti, is vandalism. I am kind of on the fence about it.
On the one hand I can see how certain works on certain buildings are nothing more than vandalism. You will see curse words scrawled on the sides of building or windows painted over or artists who have really bland and sloppy tags. These kinds of street art have little to no value and probably constitute vandalism.
But on the other hand some street art is beautiful and it enhances structures both abandoned and in use. Some of the most breathtaking art I have ever seen has been on the streets. To call this vandalism is an insult to the creators and the spirit of their work. It has to be outside on a wall because the art has been liberated from the gallery space. Public art requires public display, sometimes it just happens to be unsanctioned.