Slavery is still protected by the US Constitution

Read carefully the 13th Amendment to the Constitution. You'll see that there is an obvious caveat that permits the enslavement of American citizens who are in prison. It is being used today to exploit cheap or free prison labor for a profit. This happens in both public and privately owned prisons but for profit (private) prisons are by far the worst.

The Thirteenth Amendment (Amendment XIII) to the United States Constitution:

Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.

Section 2. Congress shall have power to enforce this article by appropriate legislation.[1]

The exception that legally allows slavery comes after the first comma in Section 1:

"...except as a punishment for crime whereof the party shall have been duly convicted..."