1) Both sides had a good reason for the original situation (US refusal to pay reparations demanded by Vietnam) (US need to get out of Vietnam ASAP)

2) Precedent in behavior (French prisoners ransomed by 'Cong after Dien Bien Phu)

3) Extensive government apparatchit involvement to deny due to liability (Several present and future presidents, CIA directors, etc etc from Nixon to Bush to Robert Gates)

4) Documented contradictory organizational behavior vs. public mission ('lost' MIA contact codes)

5) Documented US government knowledge of disparities between Vietnam returned prisoner list and US government list of MIAs - particularly Laos

In this case, I can totally see that a relatively small group of powerful politicians and bureaucrats were incentivized to make sure that such a devastating revelation would never be made - of a realpolitik decision in a traumatic period in American history.

Not proof, but Mr. Schanberg has the right attitude: It isn't that American MIAs left in Vietnam are incontrovertibly real, it is that the American government and military have been acting in a way contrary to their public stances for many decades.