I imagine it's about not only the device's signal transmissions, but its physical weight. Planes during takeoff and landing are at sharp angles with extra G-fo

I imagine it's about not only the device's signal transmissions, but its physical weight. Planes during takeoff and landing are at sharp angles with extra G-forces. If your iPhone was in your hand during this time and you accidentally let go, maybe the phone would fly at someone else with enough force to hurt them.