If the WiFi network is open (no password required to join the network) then it should be legal to use it. If you have to break encryption to get connectivity then that should be illegal.
Here's my reasoning via analogies.
Unlocked Front Door (Bad Analogy)
Even if someone leaves the front door to their house unlocked it is still illegal for strangers to enter because they would have to trespass without the owner's permission. This is why we lock our doors.
Bedroom Window Viewable From Public Street (Great Analogy)
If someone can see into your bedroom or bathroom window with the naked eye (pardon the pun) you would draw the shade for privacy since, like WiFi, it is something that can be accessed from a public street.
Think of adding a password to your WiFi network and drawing the window shades both as privacy/security.