Exactly! Theres no reason why either side cannot choose to see the crimes of their own people. And there are certainly those, on both sides, that can.
But where America has a huge judicial system that can protect it's informants and give new identities if peoples lives are threatened and law & order is, for the main part, maintained, in Iraq, they have nothing, they have men walking the streets with guns, it no longer matters who 'they' are, American or Insurgent, and you are more concerned about getting home to your wife and child, keeping your mouth shut and surviving another day.
Iraq has a beleagured Police force, and a frightened population, the deliberate attacks on 'collaborators' are designed to stress that further.
I know that at least the first beheading wasn't commited by an Iraqi. As for how many of the others were? I don't know.