Well, they are letting you on to their LAN which almost certainly includes a Windows PC, and without the protection of the NAT/PAT switch (I refuse to call it a router!), they are toast. I bet at least some of them set up WEP for encryption. Just crack it.
There is this idea that if I let you on to my WiFi and you do something illegal (download music, etc), then I will be held liable. Certainly the IP address of the illegal activity will be traced to you and you will be the first suspect. But recently (I have to find the reference to this in my archives), a judge actually found some guy guilty *because* he had turned WEP on: the judge held that since his WiFi was secure, the person doing the illegal usage had to be him. Better then to leave your WiFi open and log all client access (though of course some of that information can be fabricated later).
--ravi