The problem is here is not the kindle but the vast array of appliances people bring onboard.
As you know the critical phases are t/o and landing therefore crew prepare the cabin for such critical phases.
Therefore more attention is needed so the less you do the more you are ready for an eventuality.
The worst offenders are the music systems. People who listen to them don't hear the safety announcements and we have to ask them one by one to fasten their seat belts put their seats upright, latch their tables.....
The general concensus is to make the rule as simple as possible and therefore all electronic equipments must be switch off. We can't enter into a discussion about what an electronic equipment can or can't do.
No pax are more and more clever and decide they know better and try to recreate a rule which suit them personaly. They forget they are with up to 500 other people onboard. We can't have a rule per pax.