The US still used slavery as a source of labor up until about the mid 20th century. Southern states would do “prisoner leasing”, where prisoners were basically given to farmers and plantation owners to do labor for free. It started right after the end of the Civil War. Many black boys and men were arrested on trumped up charges and “leased” back to the same plantation owners they were freed from.
They used that part of the 13th that said “Well, except prisoners, those can be slaves.” Local law enforcement rounded up former slaves on trumped up charges and leased them back to the same plantation owners they were freed from. Only now if they escaped they were “escaped criminals” and they could count on even northern law enforcement returning them. The US is still a pro-slavery country and will be as long as that part of the 13th amendment stands.