You misunderstand. The vector is: I take random persons phone, compel them to provide me their finger print, and now when I plug the phone in to a computer, I c

You misunderstand. The vector is: I take random persons phone, compel them to provide me their finger print, and now when I plug the phone in to a computer, I can unlock the device and then tap "trust" in order to back up the phone to the computer. If the iPhone was already powered on and the user had entered their passcode after it turned on, even if it was two weeks ago (there may be a time out I'm not aware of, but you get the point), I'd be able to backup their device to my computer.