I'd regard the boardband router protected area inside a home network a part of the "home".
If you found such WiFi connection unsecured and try to connect, it's equivalent to walk pass someone else's home and found the door unlocked, then go inside the house.
It doesn't really matter if the house owner intentionally leave it open or not. If you'd not asked and there's no sign inviting you to walk in (I'll consider having "open" or "free" as a part of SSID equivalent of "welcome in" in this sense), you're doing it wrong.
Really? Because I didn't, actually, go inside the house. I interacted with the radio signal that you were broadcasting at such a high level that I could hear it well outside of your domicile.
It may have been possible to confuse the issue such that this analogy seemed logical to an untechnical judge, but it isn't logical to someone who knows how electromagnetic waves work.
If you turn on your stereo loud enough that I can hear it outside, am I 'stealing' from you by listening to the fine quality tunes you are playing?
If your ham-radio tower broacasts on the same channel as I happen to be speaking, am I stealing from you?
Or, more to the point, if you have a promiscuous unsecured wireless router that passes any connection from any device right through to your ISP, how is that theft?
It isn't, and frankly, the ISP has more of a case against the router owner than the person who connected.