According to the WaPo article linked below, "It is illegal for people to write or mark on any public or private property without a permit. The District’s Department of Transportation, which issues permits, did not respond to calls and emails seeking comment...

"D.C. police declined to comment on the allegations but said they “did not issue a permit to put a message on the street,” as those permits are issued through the D.C. Department of Transportation."

I personally don't give a crap if a group wrote that stuff on the street and am not sure why police would choose to arrest them, but for you to be fine with assaults on peaceful protesters in various places and the forceful clearing out of Lafayette Park, but then complain about this, is hilarious.