Perhaps, but it isn't part of the heritage of the rest of the country. If they want to celebrate it, that's just fine and dandy; let them fly their flag outside their homes or private clubs or put confederate stickers on their pickups. I don't give rat's ass.
It's of no concern to the country as whole, and hence of no concern to the president. Hence anyone who makes an issue of it during a presidential campaign is an idiot.