>I'm sure this will happen in the US, since you can't do what New Zealand does- which is to require airlines to ensure passengers have necessary documentation before they can get onto a plane going to new Zealand.

Maybe they do that to New Zealanders and citizens of EU and few other countries, but, believe me, in some places it's impossible to buy a one-way ticket to the US. In case they don't let you in, you must make sure they won't pay for your transportation back. And the travel agencies want to see your paperwork, or else they can't sell you a ticket. It all depends on the color of your passport.