Amazing how, with all the contrary evidence in abundance, that myth lives on.
This is just another aspect of a conflict we have long been fighting. It's about who controls the Narrative. Currently the narrative is pushed from New York Liberal Democrats, (News) and Los Angeles Liberal Democrats. (Entertainment)
In the after math of the civil war, there was no means by which northern cities could be informed to the extent necessary regarding what actually happened and why.
They were fed a media narrative, they wanted to believe that narrative, and they endlessly repeated that narrative.
Even today, descendants of Union soldiers want to believe their ancestors were the good guys. They do not even wish to entertain the thought that their ancestors might have been the bad guys.
That the war was fought to end slavery is their fig leaf to which they desperately cling. They think "it must be true, because the alternative is too horrible to contemplate."