You assume. First, I often hosed it down myself so that I could make changes and start over. Second, while my mom went all :2mad: over it, my dad was fine with it, and if anybody was ever going to "scrub the damn patio"... which never happened because it was never necessary (fact)... it would have been him.
Third, when my kids and nieces and nephews were younger, we often bought sidewalk chalk and let them go to it during family events. It never lasted long. never more than a couple days. I cannot speak to your experiences, but my direct and varied experiences throughout my ever increasingly long life say you are flat out wrong. It DOES just wash away.
Bottom line, "defacement" is weak. It does no permanent damage whatsoever, and as such this is a thinly-veiled attempt at political persecution. Especially since they aren't prosecuting anybody else who isn't making political messages. Personally, I would be ashamed to claim support of such a feeble and unconvincing stance by our so-called justice system. But hey, if it works for you, have at it. Make sure they go after the little kids doing hop-scotch in the neighborhoods, too. That's no less "defacement".