It turns out that "Free Public WiFi" is not a scam or some sort of malware, but the result of a Windows XP bug. Some versions of XP, upon not being able to find any network, will attempt to create their own network, with the same name as the last one they connected to. (Why that made sense to someone, I have no idea.) Which means that, at any time, there'd be a lot of zombie WiFi networks floating around, hosted on Windows XP laptops and named after whatever they connected to last; in other words, a broad sample of network names, which don't do anything, other than inviting passersby to connect to them, like a giant petri dish to test wireless network name attractiveness.
Of course, when someone connects to one of these networks, they don't actually get an internet connection (or anything else, for that matter). If, however, they're running an older version of Windows XP, their machine is now "it", and will next create its own network with the same name as the last network it attempted to connect to. And so, the most attractive names spread like a mostly benign contagion though the wireless spectrum, with the most attractive name being, it seems, "Free Public WiFi". (One might argue that "Free Beer" or something similar would be even more enticing, but for the plausibility gap.) Other common zombie network names you may have seen around are the default names of hardware devices' networks, such as "hpsetup" and "linksys".