Getting arrested for sidewalk chalk, even if he is saying stuff like Fuck the cops, is ridiculous. It's called free speech, and free speech means allowing unpopular opinions to be heard. There is no such thing as the right to not be offended.
Now if he was advocating violent acts towards cops, then get him the fuck out of our society.
They wonder why they're harassed so much.
A pertinent fact from the article:
The affidavit says that this was explained to Osmar multiple times, but that he disregarded the warning and continued to write on the ground.
In other words, he wanted to get arrested.
So the question is if the city prohibits the use of chalk on a sidewalk under all circumstances. Given that it's unlikely that any child has been arrested for drawing a grid for hopscotch, and we have not heard of the city prosecuting a store for having sidewalk art, then we can reasonably suspect that this law is not being universally enforced, and would therefore run afoul of the protection of the government in CITY COUNCIL v. TAXPAYERS FOR VINCENT.