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.
So you think people should be prosecuted for what they think rather than what they do? In reality all they did was put up signs without a sign permit.