Just because you can power a device up does not mean it has not been modified.
Anyone with even moderate skills could EASILY take an off the shelf business class laptop, remove 3/4 of the guts of it, replace it with a tiny SOC, fill the case with explosive, and the laptop would boot and display and work just fine. The only way to know it was modified would be to look in detail* at the system specs and compare to an online DB - do you honestly think that TSA is going to do this? Replace all of above with phone / tablet, it is the same story.
* Oh, other than XRay the damn thing, which is what the TSA does anyway do they not? I honestly do not get what this "powering on of electronics" is supposed to achieve. Only the most idiotic of plots would be foiled by this.