Unfortunately, the FAA will not allow them to be switched on while the plane is in the air for fear they may interfere with aircraft's flight instruments. hahahahahahahahahahahahah!!! That's what "Airplane Mode" is for.
Have you been on an airplane recently? Every flight I've been on in the past few years I've been instructed to turn the devices off completely "even if the device has an airplane mode."
Yes, just a few months ago on a domestic flight, and I take international flights annually. Not only have I never been asked to turn off ANY electronic device (either with or without a flight mode option), actually the use of flight-mode enabled devices is explicitly permitted and instructed during the pre-flight broadcast.
I use mobile phone for music, tablet for browsing/reading, laptop for movie playback during all time of the flight except for during takeoff and landing, there's no chance that a crew member could tell me to switch any of them off (except for during takeoff and landing) without a resulting in a compliant from me.
Not too certain about FAA rules but it's probably a US thing.