Just last year somebody had created an image of a car made in a similar way but using fallen leaves. Maybe that was a purely spontaneous act, the leaves where lying around, why not just paint with them?
I would plea that illegal urban art makes city life better. Blank walls and fences should be released for urban intervention. Streets should be proclaimed to be galleries. Street artists should be celebrated, we should thank them, they are bringing art back to the people. All the people. Not just the elite that visits galleries and museums.