Better to simply treat them as the temporary installations that they are. 

Motague himself was ready to paint over the Brown mural. The attachment to these paintings is feigned. By Montague for wanting to keep "his" space, and by the black community for yet another opportunity to play the victim.

But hey, you guys keep threatening lawsuits and see who lets you paint anything else. The entire mural scene will be reduced to a handful of professional painters, none of whom will be painting questionable political stuff.