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. If 1 person pays for something and someone else takes it, its theft.
The locked car analogy breaks down right there. If you drive around in a car, with the door open and a big sign saying "I will drive anywhere!", you are just begging for somebody to hitch a ride.A free WiFi access point is, as far as an system should be concerned, a deliberate act to provide wireless Internet access for free -- just like hitch-hiking, and that is (mostly) an accepted practise.
Think of it by the actions associated with public (unsecured) wireless Internet and stupidly set up (unsecured) wireless Internet.
Either case provides the exact same scenario, often without a sign and often without any change to the essid. How is the user to know the difference? The law should take into account that People Are Not Telepaths.