Obviously, people don't want random retards with little to no artistic skill sparying shit over their buildings.
But i would be wondering why not, if someone skilled went to city hall with a portfolio of his/her work and asked for permission, to paint some booring grey walls with something cool, that wouldn't be offensive, and was told no.
Then again, if some guy is allowed to do it, there'll be tons of retards who won't be able to understand why he can paint on the wall, but they can't smear their crappy tags all over the palce...
I actually thin more legal tag-walls would help reduce unwanted grafitti problems to some extend.