That's also not legal. You can't simply walk into someone's house, regardless of whether it's locked. It's unlawful trespass, it's a gross violation of privacy (especially since you took a picture), and it may be legally deemed breaking and entering, burglary, or some other felony depending on jurisdiction.
Also, I believe that the voicemail "hackers" also deleted some voicemails, and in that case something was indeed taken.
I don't think the analogies are doing justice. Nobody's house, not even a virtual one, was involved. An open-to-the-public voicemail system was used. The 'victim' was sympathetic and folks reacted emotionally. Its not clear to me this was espionage, or even a misdemeanor.
You're still trying to link "unlocked" and "open to the public". The fact that you don't lock something does not mean that it's legally open for anyone to use/take/whatever. If your front door is unlocked, it's still a crime for me to enter. If you leave your voicemail unlocked, it's still a crime for me to access it.
These comparisons are getting pretty tenuous, don't you think?
If you run foo.com, and you open it to the public, then you have made the decision to allow the public to use it. You've got a DNS record that tells the world they're welcome to access your site. On the other hand, if I sit outside your house and use your open WiFi to scan for computers inside your house with open ports, I'm probably committing a crime. If you have not made your desktop machine public, my accessing it is almost certainly against your wishes.
I'm thinking that voicemail is pretty public. Not as public as a web site, but more so than your desktop machine - after all, its available 24X7 from any phone in the universe.
The difference is only expectation. Some folks have the expectation that voicemail is as private as, for instance, an answering machine. Which is clearly not the case if you are technically savvy, which I'm thinking a judge is not.
To access my voicemail, you have to enter my pin. If I have not set a pin, there's still a default one. The way around this is to spoof my number (on a phone that uses pinless voicemail access) and pretend to be me. In no case is my voicemail public, either in practice or in intention.
I'm not sure how you think an answering machine is different from voicemail. I can access my answering machine from any phone in the universe, too, or at least I could if this was still 1995. Voicemail is just an answering machine than handles a lot of telephone lines.
It wasn't open to the public. Her voicemails, my voicemails, your voicemails are in no sense published. In the Milly Dowler case, the PI raked through the bins of the dead girl's family a few days after she went missing to find leads on what her phone number was.
Edit: bins == trash (sorry was thinking in anglicisms)
Also, I believe that the voicemail "hackers" also deleted some voicemails, and in that case something was indeed taken.