I would think the weirdness is because there are constitutional Due Process requirements to being able to arrest somebody, charge them with a crime, or take awa

I would think the weirdness is because there are constitutional Due Process requirements to being able to arrest somebody, charge them with a crime, or take away their gun rights. But the Feds can easily lean on the airlines to not let people they don't like board a commercial airliner. That'll maybe make us question a few things. But then I think another part of the reason is that most of the international terrorist types that were originally expected to be blocked by the No Fly List were generally not in the US, and so not subjects to US Gov charges or even gathering evidence.