The number of open non-encrypted wi-fi networks around in my neighborhood would suggest IP alone is a poor evidence.
In my experience, I can stop my car just about anywhere and open my laptop to find one.
Sure, they ought to secure their network, no doubt about that, but fact is they don't. Because of it, they should have to mach contents and computer to have a case at all, given the matched computer is in the household.
At least in my country, I would be able to crush their argument in court if IP was all they got.
There are a number of reasons why a network could end up 'Open' even if it was encrypted. A power outtake or manual reset can cause that and I would be hard pressed to say the owner should have known better as his computer may be set up to connect to any (not just encrypted) WiFi networks and so unless paying special attention, the user has internet and does not need to notice the encryption is gone.
Even if the network is encrypted, kids have friends visiting who hook up various digital 'stuff' they bring to the WiFi network, neighbors may be able to hack it and so on. There are probably even more reasons here, but it all points to IP having less credibility as evidence.