I think what is more important than why they would have slaves is the fact that they have them at all. I think KC is trying to show the ugly side of humanity. It demonstrates what would happen if society and its rules completely broke down. There would be absolute chaos. Without laws and punishments, the morals and ethics most people live by would take a second seat to survival and pure enjoyment. I don't doubt the Mallers are using the slaves for labor and sex.