I'd like to think it has more to do with the post-GwtS Fantasmic survey.
I indicated on my survey that I brought my own ears, while my earless bf indicated that he did not. The questions on his survey were geared toward reasons he didn't have one. One of the possible responses was "too expensive." I'd be willing to bet a lot of people said that.
I think that for some, it's not that they don't have the $20, but that they feel it's an issue of principle, that ear hats shouldn't be more than, say, $15. By offering it free with a $50 purchase, one feels that they're getting a deal, especially if they were going to spend that money anyway. It's more like... Now you're paying less than what you thought you should for those ears, rather than more.
And it's better than those other "deals" they run: spend $50 and "receive" this backpack/throw blanket for $17.95! Ick.