A few days ago, Kahler Nygard reportedly received a boarding pass indicating secondary screening was required, but that pat-down was reportedly never done. So when the plane landed, they took the passenger off the plane ahead of the others and requested that he submit to the screening at the destination airport, after the flight. He recorded the incident on his phone, and after confirming that he wasn't being detained, simply walked away.
The proposition that screening a passenger after the flight has landed might retroactively increase the fight's safety is, I think we can all agree, counter to the fundamental nature of causality.
But the question is this: Is screening passengers after they have arrived at their destination without incident within the rights and responsibilities of the TSA? Clearly it doesn't jive with the whole "cause and effect" framework of reality that we all live in, but are they within their rights to demand such screening anyway?