Sadly it is theft – which always seem a really stupid law. If people don’t protect their wifi then it should be tough. Or if it’s such a crime why can’t I insist my neighbour prevents their nasty wifi signal coming into my house.
Saldy the law doesn’t take my stance and can prosecute.
However, if they’re stupid enough to leave a wifi unprotected, they’re probably also too stupid to be logging, or able to identify, another user on the link.
If the link is a low speed one then they may notice a performance change and report it to their ISP – and they will be able to trace you via your IP…..