We are one of the trial airlines for paperless U.S. immigration i.e. the ESTA information is used in lieu of the green I-94W. A surprising number of people refuse to believe us when we tell them, and so we give them a green placebo to complete - which is then discarded at CBP when the present.
To be fair to the passengers, an understandable reaction given the potential humilation that can be inflicted by immigration authorities for incorrect or missing paperwork, especially if they have witnessed people being sent to the rear of the queue on previous visits.