The "open front door" analogy really doesn't work in this situation. Even if the door is wide open you have to physically move your carcass onto Private property thereby committing the crime of Trespass. It's not like that with WIFI.
Now, IANAL but, an open access point that is broadcasting it's SID is BROADCASTING an open invitation to connect, over PUBLIC frequencies, and at least in this case, well beyond the Physical boundries of the Private property, onto PUBLIC property.
The act of broadcasting the SID of an open access point could be considered an invitation to join the network and the DHCP process could be considered the act of asking for and receiving permission to join the network. Remember even if you connect to the access point, you can't use the internet connection without the IP information supplied by DHCP. Using the internet connection is the "theft of services", right? Not just connecting to the AP and doing nothing.
In US law (roughly based on UK law, right?) there is a concept called "attractive nuisance". This basically means that if you have something on your private property that is visible from public property and everyone who sees it is going to want to use and you make no effort to secure it or prevent access to it, you give up to some degree, some of your property rights to that thing and possibly the piece of property it is on. Here's an example; you are a single person with no children and you live alone in a house with a large unfenced backyard that borders a public park. One day you build a children's playset at the back of your property, easily visible from the park. some children playing in the park notice your new playset and wander over and start using it. One of the children falls off the slide and breaks his arm. His parents sue you. They will probably win even though the child trespassed and used your property without permission. You intentionally created an "attractive nuisaunce" and made no attempt to control access to it. You could have built a fence, you could have posted signs that said "Private Property - keep off - this means you little brats" But you didn't, so you accept some responsibility. It's probably different in the UK so somebody correct me if i'm wrong. But there are ridiculous laws like this in the US that need knocked down too.
The best analogy i could come up with in this situation is this;
I'm walking down the sidewalk to work one morning. I walk by the front of your house and notice that your garden hose is hanging over your fence spouting water onto the Public sidewalk. Since I'm thirsty, i lean over and *without physically touching your hose* (that's garden hose, pervs) i take a drink of water.
Now have i tresspassed? No. Have i stolen your water? Maybe, would it matter if I had let the water hit the sidewalk first and then lapped it up from there? The water was going down the gutter anyway, at what point does it change from private to public property?
As previous posters have mentioned, the guy made a huge mistake admitting to anything. The proper response when the Neighborhood Nazis walked up and asked what he was doing would have been either to say something like "I'm sitting in my private car, legally parked in public place working on my private laptop, not that it's any of your business - now fuck off", or, simply winding up the window and ignoring them. If/when an actual cop shows up you have to deal with him but the same rule applies. "My name is so and so, I live at x address, I'm sitting in my car working on my laptop. Period. If you have any more questions i'm going to need my lawyer present."
It seems to me that these kind of laws have only existed so far because they've only been applied to the uneducated. The first time someone tries this on a reasonably well funded IT professional, with a bad attitude about authority, I can't see it holding up. All it's going to require is the right expert witnesses to explain the situation in layman's terms to the jury in a way they can understand and no reasonable person would convict. And at least in the US these laws are pretty vulnerable to constitutional challenges, the problem is that it's so very expensive to take a case far enough on constitutional grounds, that it's going to take someone with a ton of money, a professional understanding of the subject, lots of endurance, and a strong enough feeling of Civic responsibility to not just take the settlement and make the whole thing go away.
At least in the US, it's government OF the People, BY the People, not just For the people. If you don't like the way things are going in your govt, Get your fat asses off the couch and DO something. Just voting for the lesser of two evils ain't enough. They're both obviously in the pocket of the global corps. If you're not willing to fight and suffer for your freedom, you don't deserve it. Maybe start by learning your actual rights under the constitution and federal and local laws and the proper way to deal with law enforcement, so you don't make it so goddamn easy to take away those rights.