To many who have had their properties damaged by graffiti, it is indeed a crime to paint, mark, scratch, scrawl on properties that do not belong to you. It's vandalism!
But if we are to think along Newton's Third Law of Motion that for every action, there is an equal and opposite reaction, then the more appropriate statement to use should be:
"Graffiti is a crime if you don't see the social and political messages in them."
What were the reasons (actions)that initiated the graffiti (reactions)? If you see graffiti displaying the same messages everywhere you go it is likely that the issues have never been resolved.
Look at my graffiti. What are you going to do about it?