there is a distinct diffrence between ordering your "loyal freedom fighters" to die for you and being in the line of fire yourself. few of iran's modern leadership have ever been shot at, let alone fought on the front lines.
they might have a few vets from the Iran/Iraq war in the leadership somewhere, but mostly Iran's leadership including amadijacannaspellhisname probably knows nothing of actual conventional battle, they've been using subversive tactics against others, especally israel, but have never been engaged in stand up fighting. they might have a few psycho special-ops guys, but the average iranian soldier has never shot at a target that's shot back.
compare that with the average combat experience of most US forces... let alone groups like the marines, Seals or Rangers.
there is also a diffrence in mind set in american forces that their Republican guard can't match. RG's are taught to hate their enemies, that their enemies are enemies of god, they serve a theocracy. American soldiers are taught not to hate their enemies, and not let fury guide their actions.
I've heard tales from some of the army guys I know of terrorists opening fire fuled by their hate of americans, and dying stupidly. these battles were in southern iraq of course, but the image of the US soldier putting a single shot into a terrorist who just wasted a clip firing on his position is a strong one.