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)
Yes, and I would add some (hopefully) "common sense" consideration.
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).
#1 clearly, or perhaps switched off below 10K feet MSL.
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.
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)