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.
Intent is the difference in a lot of crimes. When people break the law, their intent is important. If they are shown to have intent, they will not be punished for thinking about putting up the signs, but rather for actually putting up the signs with the intent of provoking the police response. In that case, the intent is what makes the action criminal.