Under certain circumstances ... slavery IS legal in the United States. Mainly, if you have been convicted of a crime - the government may use you as slave labor. That's how they can force inmates to work at jobs the person doesn't want to.
Slavery still exists in many parts of the world. It may be some or the other form. African, and some Asian countries you will find that. In India, Pakistan, China and North Korea. But it is not legal on any part of the world.