It's called American football, not football.
It is based on football (or soccer as some people like to call it, fools). Rugby (another game based on football) is actually Rugby football. It originated in a place called Rugby.
American football is based on football, and originated in America, so it is American football, like Rugby is Rugby football (and you have the Rugby football union and Rugby football clubs).
Basically, it's football because it can't be called American (unlike Rugby, which can be called Rugby), because American means something from America, whereas Rugby is a place name.
That's what I reckon anyway.