I think FA's use phrases such as these because its normally the easiest way to get people to comply. If the FA says it is prohibited or against company policy, people wine and complain or simply ignore it but if you bring the Feds in then it may get some people to comply. Same with the cockpit sensors and electronic devices and seatbelts (well some types display this info if the crew cared to look it up).

I don't think its a bad idea for an airline to prohibit putting PEDs in the seatback pocket. It is crazy how many people leave their devices onboard. It costs an airline a decent amount of money because many passengers realize they left it after the plane has already gone. Because of the risk of theft, airlines need to FedEx the device to the passenger. This costs money to the airline in shipping costs. Not a huge cost but it does add up and is a loss of money that should be avoided.

I mean it is funny how careless people are with such expensive things. A new iPad isn't that cheap. Airlines cant find it every time people lost it since other passengers on the next flight sometimes take the device.