Shirley Temple Movie- Littlest Rebel- Rebel vs Yankees?

I'm Canadian and I dont know much about American history. In this movie, Shirley is a rebel/confederate and she has some slaves. And the Yankees are initiating a civil war betweent he Yankees and Rebels. The yankees claim to want to "free the slaves"... so the rebels are the enemies. Why are the slaves of the Rebels protective of their masters if they are supposed to get freed??