I think the issue is really about whether an open unsecured WiFi connection can be considered an invitation to use the network or not. That could actually be the owners intention. For example the owner could be sharing their Internet connection with their neighbor (maybe they went halves on the Internet bill). Now this would probably violate the ISP's TOS but it wouldn't be illegal. In this scenario the issue becomes less murky because the owner is intending their network to be shared with anonymous users. The question is since they didn't put a big sign out front saying "Please feel free to piggyback my network!" does the open connection itself imply that it's OK to connect. Now in IL (My great state

) the issue is fairly straightforward. See quote below (Thanks for the link Slim).
"Sec. 16D‑3. Computer Tampering.
(a) A person commits the offense of computer tampering when he knowingly and without the authorization of a computer's owner, as defined in Section 15‑2 of this Code, or in excess of the authority granted to him:
(1) Accesses or causes to be accessed a computer or
any part thereof, a computer network, or a program or data;"
IL clearly says "without the authorization of a computer's owner" which I think indicates that you need express permission from the network owner to use the network but I am not a lawyer so I could be wrong. Although Slim's kids would be OK in IL since they include the "knowingly and with out authorization" so if you don't know any better your fine. This is one case where being IT clueless would actually be an advantage. Well that's my 2 cents.