I thought the whole point of having to prove your gadget worked is to show that it's an ordinary working electronic device that you are taking on board for your own entertainment during the flight. Anything in your suitcase will be switched off, and out of your reach, so wouldn't be a threat anyway... The same as there are other items you can check into your suitcase (knives, cigarette lighters, etc.) but can't take them in your hand luggage.
It's not the same as knives. You have to wield a knife for it to become a weapon.
There are many ways to trigger an explosive device, and very few of them require direct human interaction.
We all remember Lockerbie. The Pan Am 103 (a boeing 747) was brought down by a bomb hidden in a Toshiba radio cassette player stowed in a Samsonite suitcase checked in the hold
No human action to trigger that bomb.
The electronic devices are composed of electronic parts incased in a shell.
The new security measures are designed to check if the devices brought onboard are really what they look like, or if they are just a "shell" containing something else. Just like the Lockerbie cassette player used to look like a cassette player but probably would have failed the "power on" check. (not entirely true since at that time, electronic devices were merely "empty" and had a lot of free space inside)
Today electronics are fitted as tight as possible, which also means less wiring and less visual cues (on x-ray) to detect a non standard device.
The security officers used to be able to quiclky identify a threat, e.g. battery + wiring + suspect plastic mass ( not going into more detail, this much is explained in old offical documentaries about airline security)
Now there is less and less wiring, and it seems the explosives are more and more diffucult to detect.
So basically, security needs to make sure that an ipad is really an ipad (singling that one out)
What happens with checked luggage ?
You can't really tell if a device is in sleep mode or if it's turned off. Just like your alarm clock (even as an app on your tablet) will have a silent timer running in the background, and wake the device up once it's time.
So if they don't check devices in the hold, there is no way of telling if
1 - the device is fully turned off
2 - the device cannot be turned on remotely, or self activated.
3 - the device really is what it pretends to be.
not checking the devices in checked luggage would defeat, at least half of the point of having new checks at the security checkpoints.
bottomline, checking carry ons only would be like securing the door with several locks but leaving the window open. (I'm not saying it's useless to secure the door, I'm saying that it would be nonsense not to secure the windows as well.)