Its clearly theft, using the earlier example...if i choose not to lock my car it doesn't give anyone the right to take it.
He didn't take it. The Internet connection is obviously still there, just working slower. It would be more analogous to power skateboarding. Which (while dangerous) is not to my knowledge illegal. Your rights to your own property are not contingent on how well you secure that property.
An Internet connection is not your private property. It is the property of your ISP. Most ISPs have contracts called "Terms of service" which outline how you're allowed to use their Internet connection. In my case for instance they stipulate that *I* am responsible for all traffic from my network and that the task of securing the network is *MY* responsibility. I suspect most ISPs have similar clauses in their Terms of Service, and that it truly is your responsibility to make sure no unauthorized access occurs. With WiFi, someone else is paying for it, and you're using it without their permission, which is stealing.
Not in a legal sense. According to English law: "A person is guilty of theft, if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it". Most countries have similar laws, meaning that using a random unsecured accesspoint (public or private) is not illegal.