UA does more than that actually (though I doubt they realize it) - you can use any Google services for free in flight. I think this is because their own in-flight site uses several Google services (maps, in particular), and since Google data is served over SSL, they have no way of knowing what's inside and which connection is what.
Moreover, you can get to other sites through various Google services trickery, though it's not terribly convenient. Someone with more time on their hands could build a proxy app that would essentially allow fairly full internet access that way (I don't fly enough to make this effort).
Of course now that I posted this, UA might find out and block it (if they know how...let's see)