I think the main deal with this is that you don't wanna put your feet directly into the field of vision of another person. I'd be a bit irritated if I sat next to someone doing that and then all flight I'd be looking at their feet (and smelling their sweaty socks/shoes/feet).
And that one champ had his foot so high, it'd be basically right next to the face of a shorter person standing there. That's not cool.