According to an article at AlwaysOn

Here's the irony in Wi-Fi public access pricing: retailers can be profitable by offering free Wi-Fi as a customer acquisition tool. But when they charge for Wi-Fi access, these retailers, and the WISPs serving them, almost certainly lose money. According to a market study coming out this summer, retailers are quickly learning this lesson: up to 30% of US location owners who plan to deploy commercial hotspots in 2004 intend those hotspots to be free or free-with-purchase.

The fully loaded cost of offering free Wi-Fi access is less than $6/day. Operating a billable hotspot costs over $30/day. Half this cost comes from building or altering billing systems, plus the endless associated customer care. The millions of dollars already spent on systems to charge Wi-Fi users by the megabyte, minute, etc., will never be recuperated. Next year, authentication should become cheap enough to be part of a profitable Wi-Fi offering, but for the foreseeable future, authorization and accounting remain dangerous distractions.

...via BoingBoing