i'm trying to think of a relevant analogy here, and maybe it'll explain the (il)legality of the situation.
so what we have here is companies leaving doors to the internet wide open. it's basically the same as if they had unmarked ethernet ports on the outside of their buildings that hooked directly into their network. would it be illegal to plug into these?
so here's something close - many houses are having gray boxes installed for the phone system - the idea is that you can plug a phone into the gray box after disconnecting the phones inside of your house in order to see if any problem resides in your house or in the wires for the phone company to worry about. i assume it's illegal to plug into someone else's gray box and make long-distance calls.
so, i think under the same scope, it's probably truly illegal to use the bandwidth of a relatively open network.

questions, though:
is this gray-box law (if it is in fact in effect) applicable because of the phone company, or due to some overlying law?
and say the businesses implement encryption - who's to say when 'hacking' through encryption is indeed against the law, as opposed to just against company policy?

i wish i actually knew the answers to these questions - hopefully somebody else will :c)