I could be wrong, but as far as I am aware, even using unencrypted wifi without permission is illegal.
It may well be a lesser offence, but it is still regarded as theft of the owner's bandwidth, which they may be paying for by the kb. By using their connection, you may be exceeding their data cap (causing them loss of connection or high usage fees) & you will cause them a slow down in their own connection, if they are using it at the time. You are also committing fraud, by using their IP address to undertake your internet activities.
As I said, this may well be a lesser crime, & the owner of the wifi connection should know better than to leave their connection unencrypted. But never-the-less, you should be aware that it is possibly still illegal.