More of a risk to the broadcasting computer than the receiving computer. This is caused by a
bug in Windows:
If a laptop connects to an ad-hoc network it can later start beaconing the ad-hoc network's SSID as its own ad-hoc network without the laptop owner's knowledge.
Probably someone did once set up "Free public wifi" at a meeting, and this false network has been spreading in a virus-like way ever since!