"Give without expecting a return." How about "Act without considering the impact on others?" And "Even if they do, the finished product is impermanent." Unfortunately, the damage to the building is often permanent. Street art can be beautiful. I love a lot of posters and murals I've seen. However, the "collaborative" nature of it should begin by collaborating with those who are responsible for the care and maintanence of the site where the artist would like to create.