I don't understand the reasoning of "maybe they didn't see the signs". In my city there are something like 8 places, other than private yards, where dogs are allowed off leash and a million where they aren't. Almost all of the beaches I know don't allow dogs even if they are on a leash. If dogs are allowed, it's very clearly a "dog beach", but those are rare. To me, dogs being on leash is the default -- unless there's a sign saying the opposite, I assume that's the rule. So, I don't really understand how "I didn't see a sign" can be a defense.
Do other people live in places where it's common for dogs to be allowed off leash and where that might be a reasonable assumption?