I was using radio as an example of an existing service being free to the consumer/listener, not comparing the technologies employed. That wouldn't make any sense, as they're entirely different (as pointed out, one is one way and the other interactive).
True, radio is not free for the broadcaster. And revenue is made via commercials as well as sponsorship. With "free" Wi-Fi, I could see a mandatory app required to be run in the browser that will layer over your viewed content which you'll have to watch periodically, and anything that blocks them would suspend the Internet connection. That could probably work. But ultimately as the infrastructure becomes more optimized, the overhead costs will shrink quite a bit.