JiWire facilitates free hotspot access via commercials that must be viewed in their entirety: The idea is that the Ultramercial, the brainchild of the eponymous company, has a high enough value to advertisers because a hotspot visitor is forced to view the ad before being given access. While JiWire, a company I own a very tiny piece of, isn't disclosing the ad rates it obtains from companies using this commercial-for-free-service format, they did note a 7-percent clickthrough rate versus industry averages of 1 percent for all online advertising.
I can't provide more information than is available publicly, but I can note that it's relatively easy to look at what a hotspot operator might obtain through a roaming agreement or as their net cost for pay-as-you-go and consider whether this format might deliver more users and thus incremental revenue that make advertising the right mix.
This form of advertising is somewhat different than the 1-inch advertising bar used by MetroFi to fund its cost-free service in that the expectation with an Ultramercial is that someone is giving up a small piece of their time in exchange for a high-value item. The ad bar used by MetroFi, as one example, is background advertising that's continuous, and thus has a much lower value relative to its exposure.