This is due to the fact that the touch screens can only detect objects that are a bit thick. The screen cannot detect sharp objects and that is why it is not advisable to use the finger nails when using it. The heat produced from the other parts apart from the nails makes it possible to operate the devices and that is why they are recommended.
It also has a special heat detection so it is completely reliable on that. The surface area of your nose, tongue, and fingers, etc. give off more heat than fingernails therefore, you can only use them on your iTouch