If your device is charged you can prove it actually works; if it's not, you can say "Well, it works, but it happens not to be charged--that's why it isn't doing anything."

Presumably the idea is that techniques used to convert a laptop or phone into a bomb would render the device inoperable. It certainly seems like a harder task to make a bomb that is also a phone, rather than a bomb that used to be a phone.

Uncharged phones could be hidden explosives. To prove it's a electronic device, you have to turn it on. But it's kind of moot if you just explode the battery directly right?

Presumably, when security asks random passengers at random times to activate their mobile devices as proof that they're not some incendiary device, this rule eliminates the passenger's ability to claim it doesn't work because it's not fully charged. Plugging Yet Another Security Hole that terrorists could use!

How they plan to mitigate phones with bad batteries I have no idea- it's fully charged coming through security, but 30 minutes later, it's completely dead? Must be a terrerist [sic].

Tin foil hat meme: they want the ability to surreptitiously power on and root devices. This only works if it's juiced.

It's to prove that it's a genuine phone/laptop, and not one with the internals replaced with explosives.

If it's not charged, you can't turn it on to show it's actually a phone.