As an aside, turns out 9/10 decoy bombs and bladed weapons are smuggled onboard with no problems in tests. All the security theatre and voodoo rituals requiring passengers to switch off all electronic devices for no actual reasons and it's still trivial to hijack a plane.
Also, switching off electronic devices has nothing to do with security. The apparent reason is that it can cause issues with navigation, as was theorised after a plane crash in the 90's. Most flights these days don't even require you to turn your electronics off, or even put it in airplane mode.
I'm fairly sure the reason that they made you turn your electronics off wasn't even for the plane, but rather to ensure that you pay attention to the safety briefing.
https://www.law.cornell.edu/cfr/text/14/91.21
The reason that different airlines have different rules, is that their OpSpecs have different (and sometimes evolving) treatment on portable electronic devices, which is their way, as operators, of complying with § 91.21
(shared because I suspect some will find it interesting in a random-trivia sort of way, not because I'm arguing against your post)
IF you were a captain, responsible for a several millions dollar aircraft and for hundreds of lives, AND IF there was a teeny-tiny, extremely low probability that using a phone (or computer or other electronic device) could cause a disaster, including the possibility of a suicide act of sabotage, how would you implement in practice the Federal Rule you cited?
1) Kindly ask the passengers to have the devices switched off.
2) Seize each and every such device before boarding, and X-ray/scan each and every passengers to be 100% sure that they don't carry with them one (hidden).
Try #2 and you find yourself unemployed as a captain. Try it as an airline and you find yourself without passengers and shortly, without an airline.
Airlines and aviation authorities balance safety, cost, and convenience all the time. ETOPS is a good example of that balance evolving. ETOPS-240 would have been unthinkable at the start of the jet age.