My Mac has this. The ones who are saying it doesn't are doing it too high up. Right in the middle at the very bottom it doesn't move. I tested it on 3 computers.
It might only be newer ones. You basically have to have half your finger on the metal because it's just a small bit.
Probably just the hardware not detecting it, I doubt it's a feature.
Tried this on a mobile computer lab that consists of about 20 13" macbook pros ranging from 2010 to 2012. They ALL do this.