The fist time I have seen it as well, but on the other hand, having seen 4ever quite often, the idea is ot that far off...
That's simple: There are places where graffiti is allowed, these are the legal places. Illegal refers to the rest.
Thanks for your answers. I imagine that right now there could be several PH.D. theses being written on the social significance of the rich expression of graffiti in Munich!