By your logic, just because I didn't password protect it, doesn't mean it's legal open for you to load in a browser.
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.
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.
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.