How Can Sidewalk Chalk Art Be Illegal?

I took this today in downtown Los Angeles' Pershing Square where Occupy protesters intend to make more chalk art tonight. Last month protesters were attacked by police in riot gear simply because they were drawing on sidewalks. Apparently the legal scholars at the LAPD think the use of children's chalks made by Crayola is illegal when an adult is doing the drawing. As protests go it seems to me that chalking is one of the less obnoxious. I have read that courts nationwide are dismissing these chalking cases. Prosecutors in LA have refused to press charges against those recently arrested by our inexplicably cranky police.
It's really okay, officer! Seriously. Just enjoy all the pretty pictures and move along. It all washes off in the rain.