You miss the point, if you are told on an aircraft not to use a device by a member of the crew THEN IT IS A RULE!!! You cannot challenge it, the Captain IS the ultimate authority on the aircraft and what he says goes. If it is incorrect he may be challenged on the ground in court BUT in the air what he says goes!! If it is part of the company standard operating procedures then he has to uphold it. Just listen to it on the ground before you get on, you will hear more anyway.