It would be illegal if it were unauthorised, I'm not checking it's authorised because you'd have to be a moron to have published your open router if you didn't intend to have an open router being published. Whilst there's a chance that when I go to McDo that I'm not actually authorised and to use the published open wifi, it's so slim that it's not worth me tracking down the router owner to ask them -- if that were even possible to do.

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.

Connecting to open WiFi for internet access is just as secure as connecting to WiFi with WPA enabled. There are so many insecure hops between you and your destination that the last mile access mode is irrelevant.