Different times, different customs. The US had its own version of concentration camps - putting Native Americans into reservations while systematic killing the buffalos to undercut their subsistence. These methods were generally considered "acceptable" until Hitler perfected them a bit and used them on the Caucasian people in the heart of Europe - at which point they became unacceptable, at least for a while.
What makes a difference in the use of concentration camp approach is its strategic effect. It is quite effective when used on a guerilla movement whose main support is relatively small and isolated groups. The strategic importance of such groups for the guerilla fighters can be effectively neutralized by the concentration approach, simply because it is technically feasible to lock most of the supporters up. However, when a guerilla movement draws wide national and international support, the concentration camp approach has little strategic impact - and it becomes more of a liability than a benefit to those who run them. I think it is this lack of strategic utility rather than democratic tradition and squeamishness that prevents the Imperial Army from deploying it in Iraq.
Wojtek