Originally Posted by
stifle
Perhaps not explicitly, but VWP nationals certainly do need to be able to prove they do not have an intention to immigrate, and showing an itinerary with a flight back from the USA is part of that evidence.
And when you try to enter to the USA using VWP, you waive your right to appeal border agents' decisions. If border agent decides that he is not satisfied with the evidence you can produce, it does not help if you think the law is in your side.