If there was a risk to the aircraft, do you think that safety point would be left to the passenger to turn off the device? Of course not so if there was any risk at all of them being used they would not have been allowed on the aircraft.
The issue is that FAA rule requires the operator to determine that any allowed electronic device does not cause interference. It clearly places the burden of proof on the airline operator, that's a pretty high bar by some interpretations.
Let's set aside for a moment FAA, FCC, IATA, etc.
The airline has a several million dollars/euro worth aircraft to lose in case of a crash.
The insurance has many more millions to pay.
If I were them and if I suspected that having phones or smartphones switched on and/or used during flight might in 0.00001% of cases cause a crash, my policy would be to frisk all passengers and take all their electronic devices in custody before boarding.
Since this is not done, they (airline and insurance) must be pretty sure that the devices (and their use) are not dangerous for flight safety.
While the bomb might be a chance greater than 0.00001%, the scissors should be far less dangerous.
And this makes it risky for airlines to allow because how can the operator determine that every different electronic device will not cause interference?
I've flown small planes where my phone absolutely did cause a burst of noticeable radio interference every few seconds with LTE enabled, but those avionics likely have much less shielding than the commercial planes. Simple fix is to just turn on airplane mode or disable LTE.