Much of this has to do with the way the Civil War is taught. "Yankees" seem to be learning about the institutions of slavery, at least vaguely, as a central and motivating feature of the conflict. But often only vaguely. The economic value of human beings owned by other human beings was at the time immense, and Southern slave owners went to considerable lengths to defend the basis of this ownership as being accepted and morally righteous even. None of this is really disputable in this history. But we have a generally vague set of lessons that propagated successfully for decades, and which persist often in Southern states that this is all about some non-defined states rights or "heritage". Which makes it therefore morally acceptable to celebrate "the Lost Cause". In so far as there are benefits to decentralizing governments, the ability of state and local powers to oppress or enslave whole populations of human beings is not and should not be one of them. That shouldn't be very controversial. And it should mean we shouldn't be celebrating the people who fought or died that it should be so.
One of the facts about the war that becomes clear when it is studied more carefully is that there were wide swaths of territory within the south that didn't have very many slaves and slave owners. And most of these weren't very happy that their state governments were choosing to fight over slavery and demanding service in that cause. Multiple counties in Alabama effectively seceded. The state of West Virginia was created in this way. The CSA internally fretted over the Eastern half of Tennessee (the portion they controlled for most of the war and effectively had to rule with martial law, it was the Western half of the state that had most of the slaves and slave owners). None of that means that these weren't racist people themselves (or that the supposedly noble Northerners weren't either, NYC's race and draft riots during the war are among some of the darkest chapters in that history as well). But it does mean there were proximate causes for the war that not everyone wanted to go along with. And if that is the cause of fighting, we have to be prepared to condemn those who fought on the other side rather than just remember them or (worse) honor them. Not because they were terrible people; some were, some were products of the time, some were probably very confused. All of which happens when something terrible occurs in human history. But because what they fought for doesn't deserve our admiration or sympathy.