http://en.wikipedia.org/wiki/Prisons_in_the_United_States says, "According to the U.S. Bureau of Justice Statistics (BJS), 2,266,800 adults were incarcerated in U.S. federal and state prisons, and county jails at year-end 2010."
It seems like the United States is about an order of magnitude worse, even if you subtract the tiny fraction of US prisoners who are locked up to keep other people safe. And that's not even getting into how the countries' respective foreign policies violate human rights outside their borders, but I suggest reading http://en.wikipedia.org/wiki/Casualties_of_the_Iraq_War to start to get a handle on that.
It's not that the Korean rulers are more moral than the US rulers. It's just that they're less effective.