(It's also very, very, very terrible practice for your own security. Don't do it.)
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.
Open WiFi is like a water fountain, or a bench, in a pubic place to me. There's no explicit sign telling you to use it but who'd put it there of it were not to be used? I'm in the UK.
So, for example if I'm out and about and there's an open WiFi I'll connect to it without seeking permission .. in fact I think it would be weird to go and ask (if you could work out who to ask).