What is important here is whether the wi-fi owner is on a limited tariff. If he is then something tangible is being stolen, as downloads made by the piggy-backer contribute to the owner's monthly total. If the owner's deal is unlimited broadband then nothing tangible has been stolen.

One could also argue that anyone who can't secure their wi-fi deserves to have it stolen. But would you keep a kid's ball if it came over the fence into your garden simply because he wasn't able to keep it in his own garden?