alt Hacker NewsThere's a well known theory on this exact concept! The Medium is the Message. Or, the very act of defacing a public building is meant to sledge-hammer the artist's work into the viewer's consciousness. Compared to say, some quiet exhibit most people would never encounter.
You are not supposed to get any attention and you are not supposed to have any say in how the city and the world looks. If you buy the building you still don't get to paint.
To deface it would first have to have a face.