Take a seat
The guy made the decision not to negotiate with the youngster with his feet resting on a coffee table. He decided that he would be sitting there, and if the kid didn’t move his legs, he’d have to deal with a fully developed adult sitting on them.

The youngster didn’t react, so the guy sat down on his legs. For most people, manners are either acquired or instilled in some manner and for the young man on the subway that day, the school was in session.