But rules are a "contract" between two parties. The people who put up a sign are trying to fix a problem and they care about the intention behind the sign. But the visitors need to try to follow the rules, so they need to decide of the activity they were considering is allowed.

This ambiguity can cause issues. In a perfect world it doesn't exist. But in reality it usually does because you don't want to hire a lawyer to help you understand if the 30-page sign prevents you from bringing a toy boat into the park.