I believe the concept that is being reached for here is "intent".
If one can show knowing and willful intent to do something, as well as a repetitive behavior, then "theft" would be a fairly easy case to prove if the owner of the broadcasing device wished to pursue a legal remedy. I agree that an unsecured access point is asking for trouble, much like leaving a door unlocked to a car or house, but it doesn't change the fact that something is being taken without prior consent.
It's yet another case of "just because you can, it doesn't mean you should".
