If you place a bench in public and you don't want anyone to sit on it then you need to notify people explicitly ... it's the same, I don't find the owner of benches and ask them.
Are there any attacks that work just by connecting to someone's wifi, obviously I'm only using it for non-sensitive traffic unless it's a recognised provider, it's certainly part of my security considerations. Are there specific attacks you're thinking of? Such attacks would work equally if I had explicit authorisation, of course.
Re your link, last time I looked it was allowed to have open shared wifi, and the way you indicate it's open for sharing is having it open and shared. That's probably why the police gave cautions, it placates the complainant and they didn't have to lose in court.