The real reason you have to turn your phone off on a plane is so that you can't record the takeoff and landing. That way, if there's a crash, there's no evidence to dispute the airline's black box version of the story.
Pretty sick, no?
Send me your bill in the mail.
What's the REAL reason credit card companies don't keep backups?
Airlines are not allowed to comment on the investigation (including interpreting black box data) so the only official version is that of the NTSB.
I've never heard of an airline accident in the US where public or passenger video (cellphone or otherwise) contradicted the black box data.
People with clearances all know about this maintenance mode. None of them ever explain the error of my ways, when I discuss it with them.
This just seems like outdated and paranoid policy to me.