But if we are going over a very short distance like NFC does then their amplifier is going to have to be close to the key. If my NFC key only works when it is within two inches of the door lock, your amplifier is going to have to be within two inches of my pocket to pickup the signal to amplify it.
Well... they might have a better antenna or just a bigger amplifier than your car does. And if they can just walk past you to unlock your car, that's still pretty unnerving.