For the ten-thousandth time, it's not because your Funtendo or SmartFone will cause the airplane to suddenly explode on the runway. It's because the FAs want the smartphone addicts and social callers to _shut up and pay attention_ to the safety briefing.
Go on and on about how you've seen it before. I'd still wager that 80% of people on every flight in the US have no idea where the nearest exit is if they aren't allowed to turn their head.
In Denver a couple of years ago, people were jamming the asles with the airplane ON FIRE, off of a snowy runway in order to...get their laptops.
Given the ridiculous state of air travel and attention spans today, I'd say getting people to shut up and pay attention might be one of the toughest jobs in the world.