Your focus is on public networks though. While the ratio of unsecured private networks are slowly dwindling, there are plenty still out there. It could be argued that it's the owners responsibility to secure the network, just like it's their responsibility to lock their doors at night - but there are plenty of people not tech savvy enough to know how to do this - or to even distinguish that leaving your network open is equivalent to leaving your door unlocked and possibly wide open.
The problem is - are they wrong in assuming that their personal network should not in normal circumstances be invaded? Just because we know there is a risk of someone entering our house far easier if we leave the door open, they should not in normal circumstances be entering without obvious intent to do something, anything. The same could be said for someone going on to an open network. While it's easy enough to join said network to just use free WiFi, one could also be snooping on communication, or other possible actions on the person private network.
My point really comes down to reasonable expectation of privacy (with possible complete ignorance on such matters) from the network owner, and the possible intent of the person on the network. While it might seem like nothing, I think it's an important topic if we are discussing analog laws in a digital world.