Or, more to the point, if the Confederacy was so evil for defending the right to own slaves, doesn't that by implication mean that the United States at the time of its founding was equally evil, since slavery was legal in most states and many of the Founding Fathers owned slaves? That's certainly what the Left wants you to believe: they start by slandering the memory of the Confederacy, because that's an easy target. Once you're numb to their politically correct interpretation of the Civil War, you're halfway to buying into the rest of their agenda, which is defaming the United States and Western culture as "racist" and "oppressive" in general.
This is why you don't have to be a neo-Confederate (I am not) to recognize the campaign of vilification against the Confederacy for what it is: the first front in a PC attack on the US and Western culture generally. The unacknowledged agenda is basically "Down with Robert E. Lee and Jefferson Davis today. Down with George Washington and Thomas Jefferson tomorrow."
Virtually every nation, including the migratory nations, had slaves or the equivalent, at various times in their history. The sanctimonious denunciations of fellow Americans, who did so, is as you suggest, merely a means to an anti-American end.