Okay, I want put forth a psychological issue here. People who have been enslaved by others usually think that it's 'okay' to enslave. Their ego don't allow them to think that they have been enslaved, but rather, they think it is the way of life. So they find it 'normal' to have slaves around. Also the people who are too weak to block off the negative side of the society and accept the negatives as 'normal', think that the wrongs are rights.