That's a pity Reggie. I don't know what the legal ramifications are. But it seems pretty mean and petty. I imagine the police only acted because the city didn't want to offend paying customers (festival sponsor or organisers). But I guess it is like other restrictions often put in place in public areas (parking, concession vending, etc.) during events.

I should think the contract with the festival organiser would have had to read something to the effect that no non-booked performances were permitted in the grounds hired by the organiser. That contract should be available to the public. Might want to check it out.

Hope you don't have this problem again.