How/why is preserving symbols of the oppression of an entire race of people important?

Simply because people who cherish them see them as honoring a half millions men who fought and died for duty, not slavery and for the millions who suffured the hardship of war and survived, but not unscathed. The issues involved were political not military and that's why empathy and wisdom created those memorials as memory to preserve.
We who know war have no problem with the memorials, but with the phony attempts to divide the country again!