Ryan McGinnis
EF5
It is regulated by the power and frequency that the unit can use. In the 2.4GHz band you're not aloud to exceed 30mW of power for any channel on WiFi.
About the broadcast signal, yes it is. But, just because you can receive the signal does not mean you can transmit to it. It's no different than listening to a private ham radio repeater. Even if you have your license, you need permission to transmit on it.
Ultimately the bottom line is you're using something you didn't ask permission for.
Ethically, you might have a good argument. Legally, wardriving (with bandwidth leaching) is ambiguous. Using someone's open bandwidth is not against the law anywhere that I know of, unless the router is encrypting the signal and some sort of attempt was made to break the encryption.
Windows XP is configured out of the box to just connect to whatever the hell it sees. If you made this illegal, you would have hundreds of thousands of people unwittingly commiting crimes every day just by powering up their laptops.
Now, if you do something illegal with your wifi connection, like erasing the person's hard disk or hacking into the White House press release website and replacing it with a Youtube clip of the armless knight from "Monty Python" proclaiming that it's only a flesh wound, then yes, you could spend some time in the pokey. But thus far I don't believe anybody's been given a 300 pound teddybear just for logging onto an open wifi connection.
This should be an interesting case to follow if they actually take it to trial, since, as far as I know, it'll be the first of it's kind. I am not a lawyer, of course, and you should consult one if you need real legal advice.