The correct answer to this open question is the following.
The underlying motive for the 13th amendment leaving a loophole for slave labor was the possibility to punish people who had committed any crime with slavery.
When one reads the 13th Amendment to the United States Constitution, you notice the above-mentioned loophole: "...except as a punishment for crime..."
This loophole has been used in the history of the United States to send people to jail and punish African Americans in periods of history such as the case of the Jim Crow legislation and the Black Codes in the southern states.
The 13th Amendment was ratified in 1865 and was one of the three amendments that were passed during the period called "Reconstruction" at the end of the American Civil War."
