Well I just read up on these laws, and they really need to be updated for the wifi age. If you really want to get technical, the unsecured wifi router is acting as a proxy "doorman" for the owner.
The "plug and play" argument as to why alot of networks are unsecured is irrelevant. There is an acronym we use around here that is fitting: RTFM!
For example:
My laptop wifi starts up and seeks wifi access points by query. When it encounters an access point, they briefly communicate - names are exchanged, settings are exchanged, etc. An unsecured router is technically saying "Hi, my name is so and so. My door is open, feel free to come in". Now the SECURED router says this: "Hi, my name is so and so. My owner says that you have to give the password to come in."
The whole basis for the legality of using another's wifi is whether it is UNAUTHORIZED or not.