Warchalking (the process of finding open WiFi spots and documenting them) has been going on for a long time, but running open WiFi can pose a security problem... Not only to your computers, but any illegal activity could be traced back to you via your ISP.
The bandwidth charges wouldn't be as much of an issue in the US, as we typically have flat-rate* service except for mobile. *Of course, there are always the seedy ISPs who secretly have a bandwidth limit, or vague terms the allow them to impose any bandwidth limit on a whim--when it becomes "degrading" to their network performance.
If people really want to share their WiFi, there are appropriate firewall/gateway devices that offer some protection, but they are not cheap as they're generally designed for businesses offering WiFi to the public.
