Joining their network and using the internet or anything similar to that is akin to going into someone's house and sleeping on their couch while they're not home.
The problem with your analogy is that they are transmitting beacons frames without the privacy bit set. This frame basically says: "Hi I am a WiFi network, here is my name and all the other info you need to connect to me if you want to!" If you set the privacy bit it basically says: "Hi I am a WiFi network, here is my name and all the other info you need to connect to me if you are authorized!"
In the case of your analogy, the beacon frame sans privacy bit is the eqivalent of posting a sign that says: "Feel free to sleep on my couch if you want to!"
Whether or not the default mode should be open or private is another debate, and I understand that most new consumer wifi equipment has been addressing this from the should private point of view for quite a while. (see WPS)