Americans don't even understand the distinctions among Christian groups. If you actually went inside a Methodist church and asked people how their doctrine differs from Presbyterians' (or vice versa) you'd get mostly blank stares. So it should hardly be surprising that they don't recognize divisions in other religions. What they do understand is countries, and the US relationship with Iran has long been marked by enmity on both sides. Also, Iran's relationship with Israel is even worse, and that alone would ensure US enmity.