The airport decided to unleash its service at no cost: Denver (DIA) is an interesting case, being one of a few early airports in which Wi-Fi was built by Nokia when it thought it might become a Wi-Fi infrastructure player. As I wrote in Nov. 2001 in a story in the New York Times, the airport had a fully functional system and couldn't find an operator. Nokia was, in fact, paying them a promised fee even though it wasn't providing access. The next summer, they were lit up.
None of the country's or world's largest airports offer free Wi-Fi, while many still large but second-tier airports charge nothing, like Las Vegas, Sacramento, Portland (Ore.), and Phoenix. Denver was seeing about 19,000 to 20,000 monthly sessions at a fee, and now sees 10 times that number--without really advertising the fact until today that service was available at no charge.
I don't see this as a trend--there are typically too many expenses and too many contracts involved in providing large-airport Wi-Fi for the costs to be subsidized. Denver may have chosen to go free as a competitive move against Salt Lake City, Dallas, Chicago, and Minneapolis to get passengers from those hubs. The airport's explanation is rather thin.