Just asking

How is it that certain ideologically-driven elements of American court historiography (usually and unconvincingly self-describing as conservative), after rightly decrying the brutal and inhumane practice of mass human sacrifice by the Aztecs, then go on to excuse American race-based chattel slavery in historicist terms as a temporary glitch and a product of the times (and hey, everybody else was doing it too!)?

Amongst such ideologically-driven elements, one must wonder: do such historicist and moral-relativist excuses apply only to Americans and their forebears?

Again, just asking.