The question is not whether it exists, but why it exists. The answer can be found in the "Treasury of Virtue". Others have already posted about this. It is the false belief that the sins of racism and slavery of the North are forever forgiven because they freed the slaves. All blame must be shifted South for the mistreatment of the black race yesterday, today, and forever. It is nothing more than self righteous manure. The heart of the argument is that the Union North fought to free the slaves (which is nonsense) and the South fought for slavery (which is also nonsense). Slavery was merely a subset issue in both instances. It was present, but there were much weightier matters taking place.
These ideas are promoted by elite academia which makes things much more complicated. Modern, elite academia is obsessed with racism, and the Civil War falls prey to it just like anything else. People like James McPherson are the worst offenders of promoting this narrative. It's why so many people believe in it. He is a decorated historian after all. Historians never have a hidden agenda, right?
McPherson is notorious for dropping these ideas into his book prefaces, shamelessly, with sketchy evidence that is rarely balanced with a real contrasting viewpoint. His first page preface for Battle Cry of Freedom could be a bible for the Treasury of Virtue. But hey, he won a New York Pulitzer for it. He knew what he was doing. He gave em' what they wanted.
As long as popular historians and elite academia parade the glorious, moral, Republican and Liberty Parties in their works, people will eat it up like sheep. It's too boring for most people to try and figure it out for themselves.