I think part of the problem is that it's difficult to remove graffiti from remote areas so the groups take advantage of that, essentially creating a war over which group's graffiti wins. NYC back in the 80s spent $32M to clean up graffiti from subway cars. I doubt there is a fund dedicated to clean anything up in the forests, although like I said above I could have sworn somebody painted some of the rocks with a natural color to cover up some of the vandalism, or maybe more appropriately scrubbed it off.