It is strange that the US military in Iraq/Afghanistan has stricter use-of-force rules than American cops do.

Well, bad use of force in a foreign land and in a war situation can have bad repercussions (diplomatic but also making enemies out of friendlier locals, etc), so army guys have to be more careful and professional about it. You don't mess with such things.

While cops can kill citizens with relative impunity (especially black ones).

You would think, though, that killing your own citizens is even worse. And yet they're getting away with it. I'm absolutely baffled that this is allowed to go on in the US. Any other country would revolt over this.