Lots of places do this if they have free wi-fi. I think Denver Airport does it (or used to). The Portland city wi-fi thing used to do it. It's annoying, but they have to make a load of cash rather than actually just charging me a few bucks, amirite?

I'd pay $2 to not have to deal with the hassle those ads cause. Easily. And that'd surely be more money than they're making off of me with them.