if it is allowable on the flightdeck there appears to be no sound reason to impose a ban on use by passengers
This is not valid logic. Flight deck crew and passengers are not subject to identical rules and regulations. Many examples of this and the reasons behind it should be blindingly obvious.
Some of the reasons behind the restrictions on the use of electronics in the passenger cabin include (but are not limited to):
- Distraction of the passenger from the safety briefings.
- Lack of situational awareness of passengers during safety critical phases of flight due to concentration on electronics, rather than their surroundings and cabin announcements.
- Physical risk to other passengers from loose items flying around the cabin in an emergency.
- Objection of cabin crew to being filmed while they are trying to do their job They are there for flight safety, not for performances.
- The terms and conditions of carriage on the passengers' ticket or contract.
- The unknown threat of interference to aircraft systems.
None of these apply equally to the flight deck. You'll argue that the last one does, but if an electronic item in the flight deck causes any sort of problem it can be switched off immediately. Not so easy when the item is in an unknown location in the cabin.