I completely agree on GPs statement being absurd post-hoc rationale, but interestingly, this rule is actually coming from the FCC, not the FAA [1], with the FAA only indicating support for it [2].

The motivation is supposedly cellphones at high altitudes interfering with far-away cell towers that are reusing the same frequency at a distance that would normally make interference very unlikely, given the radio horizon and everything.

I highly doubt that that would still be an issue modern networks couldn't handle (also given that most other countries don't have a corresponding law on their books, as far as I know), yet here we are.

[1] https://www.law.cornell.edu/cfr/text/47/22.925

[2] https://www.faa.gov/documentLibrary/media/Advisory_Circular/...