For me at least, the "outrage" has nothing to do with the morality of the act. It has to do with someone who is a de facto representative of me acting like a complete idiot.
It's like walking down the middle of a street in Mumbai wearing a t-shirt with an American flag on the front and "Buddha is a fat bastard" on the back. Yeah, it's pretty much true and is not immoral, but anyone with a clue knows it's going to incite.
I see this as a valid analogy (sans the 'incite' part if that implies inciting violence; the Taliban and the al-Qaeda apologists and supporters in Afghanistan already want to kill as many of our troops as is possible and they will when they have the opportunity regardless of what type of wider justification they think they have). These Marines have shown themselves to be classless morons who do not have the least clue about what exactly our greater mission is in Afghanistan (even though that mission is wholly un-achievable). Whatever key leaders were involved in said act should be removed from their leadership positions.