I've always been pretty suspicious of the networks labelled 'Free Public Wifi' that seem to spring up whenever I open my notebook in airports, and mildly curious as to why they're so prolific. Sure, I've occasionally tried one when there's no other visible network and I'm keen for connectivity, and it turns out that's the whole problem, as Lisa Phifer at TechTarget explains:
After you deplane at the next city and open your laptop, your Wi-Fi card tries to automatically reconnect to "Free Public Wi-Fi." Others sitting around you see this SSID being advertised by your laptop, and try to connect to you. This cycle repeats itself, spreading "Free Public WiFi" SSID from traveler to traveler, city to city.
While it's hard to break the habit of wi-fi slurping, it's probably sensible to give it up, or at least follow Phifer's suggestions for not automatically connecting to unknown or public networks.
Beware 'Free Public WiFi' when searching for a net connection [Tech Target]