A common stereotype in western cinema. As opposed to Japanese ninja, an American one will perpetuate the general attitude toward far eastern culture with high-pitched screaming and ridiculously flashy moves, and should a plot require it, an intense one-on-one katana showdown in broad daylight. Will also wear a black bodysuit no matter what the lighting conditions or scenery are. Completely contradicts the entire point of the ninja in the first place, which is to assassinate in complete and total secrecy.
Bruce Lee comes to America, acts, and all of a sudden every Asian on the planet fights like him; thus was born the American Ninja.