My issue is that its impossible to determine who is going to be held responsible and for what amount of the clean up they are responsible for. Its impossible for a court to get everyone who was there, who knows exactly who was there as they came and left, and say "OK we'll divide the total number of people by the cost of damages, and call it a day." Thats not how law works, you have to be found individually guilty of a specific act which violated the law, as in "You through that specific cup on the ground, that is littering on public property" and through analysis by the court and clean up workers, they'll determine what portion of the total amount that specific person owes. If it sounds crazy for one person, imagine doing it for hundreds of them. If the government actually went ahead of this they would be wasting a large amount of money in court and lawyer fees, not to mention still having to pay for the cost of the clean up. After all the city isn't going to set the garbage sit there until the case works its way through court, they'll clean it up at their own cost and then seek damages.
I entirely agree that those who damage public property should be held accountable for it, but I disagree with the idea of punishing en-masse when a certain individuals actions cannot be proven nor can it be proven he was responsible for any damages. Individual accountability MUST be achieved before I'd support any fines being levied against protestors, it'll be a sad day in America when people can be found guilty by association.