Radio frequency, RF, interference can be a difficult thing to nail down. Sometimes it is easy: in a past life, I set up a radio transmitter at the BAFTA awards for the camera operators and the event host to hear the director, but when I switch it on, interference lines appear on the big picture monitors on the stage. Damn, my transmitter RF output is getting into the picture monitors. If I move the transmitter and aerial further away from the monitors, the interference will reduce or disappear.
Sometimes it is very difficult to track down. What can happen is that two devices transmitting on different frequencies can generate a third frequency, and the third frequency is the one that causes a problem.
Airbus aircraft have RF shielded cable looms, I assume modern Boeings do too. However, when flying auto-lands in low visibility conditions, we do so to much more critical and lower decision altitudes, say 22' or even no DA, so it is very important that the ILS receivers are not subjected to any interference and the aircraft is exactly in the centre of the ILS 'beams'.
So since it cannot be known whether every passenger transmitting device is actually benign, the safest solution is to switch all devices off completely - especially since even devices that are on receive only can nevertheless emit RF, owing to the way receivers work.
As has been mentioned, EFB's in the cockpit that can transmit data have to be tested and approved for not causing any interference.