I recall reading somewhere that the rule comes from a specific type of electronic equipment in a specific location on a specific model of plane. Here’s how I understand the story:
Once upon a time, consumer electronics were just barely at a point at which they were becoming portable enough to bring aboard a plane — portable TVs, for example. This was long before the FCC had regulations about electromagnetic emissions from such devices. Airplane pilots found that, on certain models of planes, if these devices were used by passengers who were seated in seats that ran near the plane’s communication equipment, there would be a slight buzzing effect audible in the conversations with control towers. As a precautionary measure, the FTA banned all electronics in all seats at all times, because of course that’s the reasonable response to such a circumstance.
If this comes from a long-ago case, it definitely needs a re-look. Modern cell phones have lower wattage than the old analog ones.