If a graffiti artist believed shitting on a lawn was art, they would, but they don't.

The problem and solution are similar to OSS:

The problem: the artists have something to say, they want as many people as possible to see it and use it.

The solution: make it free, and put it where as many people as possible can access it.

Yes, I just compared graffiti to github.

If there were community areas that were designed for painting, that would be totally fine by me. A big wall that is painted white, maybe with some ladders nearby if that doesn't violate health and safety rules, and tell people to go nuts. Though you would potentially get a lot of disagreeable content, but I suspect that they would quickly get overwritten anyway