If I put one bullet in a revolver, spin the barrel, point it at your head and pull the trigger, it's "No Blood No Foul" if you happen to get lucky?
So tell me again, besides possibly not getting a license to put up the electronic sign, what law did they break?
Why is it their fault the police over reacted? They have gone ape before over briefcases left on park bench's. Did they charge the absent minded person for leaving it there? If so what was the law they broke?
Unfortunately, they probably broke somekind of an unknown aggrevated littering ordinance, and will get convicted for that.
Ultimately, it comes down to intent. If the state can demonstrate that they intended to cause a police response like this, they surely broke the law, and should pay a serious penalty.
If I had my druthers, ultimately it would come down to a question of intent. But as I said above, they'll probably wind up with some kind of an aggrevated littering infraction that does not require such proof, which would be an unjust result, IMHO.