Nikomikiri said:
I always wonder how people manage to do graffiti on some places without being seen.
Who says they're not seen? I wouldn't bother reporting a graffiti artist. I only think we should paint over bad, uncreative graffiti, and we won't be able to tell whether it's bad and uncreative until after it's FINISHED. If we interrupt them in the middle we'll never know.
I bet a lot of people think the same way, or don't even care that much. Cambridge is famous for its urban murals; some of them were deliberately commissioned by the building owners, but I bet not all of them were.
As long as the artists aren't seen by cops they'll probably have time to finish up and get out of there before anyone's motivated to do anything.