Actually, an active WiFi antenna in your laptop is a sniffer, and it will tell you which networks are secured and which ones aren't. Access points broadcast their availability to all who can receive the broadcast (which is why an unsecured network should be able to be logged onto legally) and your laptop is actively seeking any such broadcasts.
Still, the laws are written to state that you cannot access a network without the owner's permission. One of these days some semi-intelligent judge will realize that places like Panera who broadcast a publicly available network, without asking for so much as a password, is giving their implied permission. Many hotels, for example, offer free WiFi, but they'll hand you a password at check-in, which effectively prevents people from sitting in the parking lot and snarfing free access.
It's rare for someone to be arrested for using a WiFi network unless they are doing something illegal online, though. Getting online from the parking lot, even every day, and checking your e-mail is pretty harmless.