Spirax, in the twisted paranoid mind of American security consultants, the fact that a passenger might know exactly where the aircraft is, via GPS, is only a very short step from the passenger seizing control of the aircraft and directing it, via his little GPS, somewhere harmful.
Except that I'm not too sure that someone who is seizing control of an aircraft will worry tooooo much about obeying the request to turn all electronic equipment to flight mode
Besides when they take control they can always just use the one in the cockpit