I'm no networking expert, but don't they also note the mac addresses and generally use DHCP for open wifi? If so, they could totally identify folks with that info...
dhcp gives addresses to the computers connected to the wifi. but those addresses are only used on the "user" side of the router. packets going out from the router to the internet do not contain those addresses (nor do they contain each user's mac address).
as far as the rest of the internet goes, there's a single address, with multiple ports, which is your router. the nat (network address translation) in your router maps from the port used to the local mac/ip (dhcp) address. so someone "outside" does not know, without additional information, which user of the wifi is making a particular connection. hence this story.