Why do Yanks call Rugby football?

Answer 1

Rugby and American football are two completely different games. Americans refer to rugby as rugby, and American football as football.

Answer 2

The game is actually the same as rugby, because of slightly different rules, but is commonly known as American football.

Answer 3

No. Rugby is refered to as Rugby or Rugby Football. The latter being the name of the game in its earliest years when the Rugby game was derived from football (soccer)