Free public Wifi hotspots have to be paid for somehow and anyone who thinks that they are getting a free lunch is seriously deluding themselves. However there are rules to be followed and it looks like AT&T have been caught playing fast and loose with those rules on its US hotspots.
In this case the company has been injecting adverts as overlays or replacement ads on sites browsed from its free hotspots without asking permission or explaining to users that they will be doing this when they sign up for the free services. Pretty sure that's prohibited by US law.
I should say that if AT&T were transparent about what they were doing and didn't interfere with site's own ad service, I'd see little to complain about here. If someone were to offer free Wifi in exchange for showing me ads on the pages y view I'd probably accept. At least I'd be making an informed choice.
On the other hand its a good advert for HTTPS which prevents this kind of man in the middle injection.
Full details of the misuse of the Wifi network can be found here at Web Policy, the blog of Jonathan Mayer,