If the bad person isn't in the US there's no need for US airlines to maintain a list of them which prevents them from getting on a plane, but then just sends them on their way to arrange other travel plans. What we want for really bad people outside of the US who should be arrested on sight at airports are warrants. Signed by judges. Not secret lists with no oversight or transparency, and not sending dangerous people back out into the American population.

Someone who is apparently so bad they can't be allowed through airport security into an aircraft, but only if that aircraft is travelling to or within the US?

Yet someone who is apparently so innocent they can't be arrested/detained/charged with anything, anywhere, and so are free to travel the rest of the world.

It sounds ridiculous even just typing that out.