Given that Afghans have been rather casually and routinely committing these kind of horrible war crimes against other Afghans non-stop for what, centuries, before American soldiers showed up on the scene, it's perhaps best for the US to keep in mind the following:

While it would be nice for the Afghan government to be comprised of enlightened, deep-thinking Jeffersonian democrats, the US shouldn't really give a damn who runs Afghanistan as long as they aren't terrorists bent on killing Americans. It's not our business. It's the Afghans' business.

Karzai, the Northern Alliance, and other elements of the Afghan government are NOT our friends, they're NOT our buddies, and I personally wouldn't trust them as far as I could heave them on a good day. They're simply a slightly less savage collection of Afghan barbarians compared the last bunch in power, and they've promised not to support international terrorism aimed at killing Americans. I guess that's good enough for starters, and good enough for us to do business with them.