But it seemed quite the epitome of efficiency, and quite incapable of being misunderstood, when the Flight Attendants told us "anything with a switch" had to be off when we took off and landed.

Why make this any harder on the flight crews or give Joe & Josephine Citizen any "wriggle room" about the rules?

It's like a few weeks back, when they tried to change the rules on the length of knife that would be acceptable, then reversed themselves and said no knives at all.

that a low-power e-reader could somehow screw up an aircraft's systems, yet a screen in every seat back going on and on from the second you walk on to a plane seems to have absolutely no effect.

The manufacturer goes through a lot of effort to ensure those seat back screens don't emit anything that will affect the instruments, including when, a decade or 3 down the line, those units are getting old and cranky. That's part of the reason a new airliner takes so long to certify for use.

Further, they are something the pilots have direct control over. If things up in the cockpit get wonky, they can pull a circuit break and all those screens go out, right now.

However, they have no way of knowing what a made-by-the-cheapest-bidder MP3 player or knock-off game system you may be bringing aboard may do. A lot of those devices couldn't care less what stray signals they emit, so long as make the proper boops and beeps and make pretty enough pictures on the screen. And if one of those makes a horizon indicator flip on its head there is no circuit breaker the pilots can pull to shut it down when they need it shut down, ie: NOW.

Just went through all that shit today, it's crap. The TSA has never caught a terrorist, and we still act like the jihadis are going to seize planes to slam into buildings. It's never going to happen again with commercial aircraft.

But, let's treat everyone like a potential criminal simply to justify TSA jobs!

As Ready4Change mentions above, the on-board systems are shielded.
It has been shown that electronic pax items currently have little or no effect on flight instruments and navigation.
The primary issue is safety.
Having those items depart the owner during a sudden unforeseen collision with mother earth, can cause injury.
Laptops can cause egress hazards. Ipods and such, not so much.
That's assuming you survive the unexpected stop.

My cellphone at 200 MPH, or a hardcover book at the same speed? One's legal to look at during takeoff and landing, and one's not.

It's all about control. The airlines have been given a piece of the control-freak thing that the TSA loves to do, and they won't let it go. That's the reason they freaked over the "small knives" thing a few weeks ago, which got a couple of corkscrews taken from us on a previous trip, because they had one-inch foil cutters. Even a French airliner has not been hijacked with a corkscrew.

I have no axe to grind on the policy. Ban 'em all, or anything goes, or anything in between wouldn't affect my life much or hurt my principles. But....it drives me nuts that every discussion of this issue seems to involve lots and lots of speculation. Can we not get the folks with the oscilloscopes and the clipboards to give us some actual facts here?