They're not "rewriting" history but putting one generations' interpretation of the past in the context of current knowledge. There is no one objective "history" just waiting to be written down by us. People produce histories according to their own needs out of the events of the past. 

How do you think black Southerners feel about statues erected to men who fought to maintain the enslavement of their ancestors? One highly doubts that they see these statues as "ancestors who died defending their states from invaders."