[addandsubtract]

If someone is planning on triggering a bomb on a plane, and they haven't done so, you can assume they have a target you haven't reached yet. So going back is not only the safe option, but also the location the people & plane came from.

[rbanffy]

The only thing it protects is the target. If there is a terrorist on board and they expose the fact they are aware of the bomb, or the bomb is minimally capable, the plane is doomed whatever they do.