As much as would love to agree with you, I don't ever think it would hold up in court.
It is not about what would hold up in court, it is about removing them from the scene of the protest...or at least disrupting them in the process.
I'm not going to look it up, but i think the US Supreme court has already rule on this one.
I know the Supreme Court has already ruled on this. I have looked it up. Enough arrests on state charges and the cost incurred in the process of appearing in court to answer the charges could break them financially. Problem potentially solved.