The artists are the ones that actually have some meaning, try and make a statement, or simply want their work to be seen by others. These people use a wall as their canvas and a city as their gallery, and regardless of your moral opinion that should be considered art.
Tagging and doing it for vandalism on other hand is NEVER art IMO no matter how good it looks.
"Music is an indirect force for change, because it provides an anchor against human tragedy. In this sense it works towards a reconciled world. It can also be the direct experience of change. At certain points during some shows the reconciled world is already here at least in that second in that place. Operation Ivy was very lucky to have experienced this. Those seconds reveal that the momentum that drives a subculture is more important than any particular band. The momentum is made of all the people who stay interested and keep their sense of urgency and hope."
- Jessie Michael, Operation Ivy