Originally Posted by
GregMM
the reason that you have to turn them off *at that time* is so that they can verify that you've turned them off *before they have to take their seats for takeoff*.
Moreover, I'm sure that there is some subset (possibly quite a very large subset) of all electronic devices that work just fine on T/O and landing. The problem is that unless the FA has a degree in electrical engineering (and maybe not even then), it isn't clear on visual inspection if it's an electronic device that's emitting some massive amount of E/F or if it's like most electronic devices and is operating with minimal E/F. So they all have to be off.