There is NO WAY this is "fair use and free speech". You can't even make that argument with a straight face.
It has been argued before http://www.nyulawreview.org/sites/default/files/pdf/NYULawRe...
If there's a case to be made protecting graffiti on private property I would say there is a case for protecting graffiti on public property if it is the public's will to do so http://repository.law.umich.edu/cgi/viewcontent.cgi?article=...
EDIT: o_o