As a southerner, its honestly pretty simple. The people of the South fought one truly great war as the South. The fact that it was a war fundamentally for slavery, is an aspect of the culture and specific history of Southerners, and obviously colors the controversy, but is mostly immaterial to the why.
Southerners, like all people, fondly remember the time when they were at the most powerful, their most ascendant, their most energetic. All people memorialize their dead, even entire generations when a great war is lost or won, and the nation with it.
The people of the South were slavers, who gave up their mastery not willingly but by fact of force. Whenever a People is cast from their own power, and subsumed to a larger entity, their self-idealization tends to become frozen at the point were they were last in control of their own course and powerful.
Is it treason? Here and now? No. Southerners are not trying to secede, and even the people I know who are the "visible" supporters of the Confederacy, are not even remotely serious about it.
It's distasteful, certainly, for one's culture-heroes to be a generation of slavers pulling their countrymen along to a bloody war to preserve their grotesque way of life, but if that's you're history, that's your culture. Attempting to force or cajole Southerners to give up This aspect of their culture will only prolong the struggle. These heroes can only be replaced by new ones, and unfortunately the South is largely devoid of heroes.