An acquaintance used to work airport security and he told me they asked to unlock/power up etc. electronic devices such as phones, laptops or SLR to check if they are real electronic devices. Apparently they were -- maybe are -- afraid of EDs that looked like electronics on the scanners.
Not completely groundless, even if I could think of more than one way to construct a device, e.g. laptop, that boots up and still explodes, but hey.
Perhaps EDs are a concern, but mostly what terrorists actually use phones for is remote detonation. A feature phone is the ideal sort of radio transmitter, modified or not, to send a remote command to detonate explosives. If security is really scrutinizing carry-on luggage, then perhaps the strategy moves to placing a bomb in checked baggage, and carrying only the trigger through the checkpoint.
So if you force someone to turn on their phone or laptop at security, you will hopefully force the decision point; even if the inspector cannot tell the difference, a terrorist is going to get real nervous and jumpy around activating the thing they intend to use as a detonator, in contrast to some businessdude activating his very ordinary mobile device.
Not completely groundless, even if I could think of more than one way to construct a device, e.g. laptop, that boots up and still explodes, but hey.