You would think it works the way it is written but there have been a few instances of flights being diverted even though they would never fly through US airspace. And don't even think about flying to Canada, they are in lockstep with the US as far as the no fly list is concerned.
For example, last year a flight from London to Mexico City was turned back because a US citizen on the no fly list was onboard. Plane landed back in London, the passenger was asked to leave, no argument, and the flight resumed eight hours later. No one has been able to find out who he was and what happened to him; arrested, cleared immigration, deported (where?).
For example, last year a flight from London to Mexico City was turned back because a US citizen on the no fly list was onboard. Plane landed back in London, the passenger was asked to leave, no argument, and the flight resumed eight hours later. No one has been able to find out who he was and what happened to him; arrested, cleared immigration, deported (where?).