...I have used the analogy before of a farmer who treats his cattle kindly, but in the end, slaughters and eats them.
It's simply that some traders and owners practiced good livestock management, which increased their return on investment. NBF was nobody's fool, he was a self-made man that was really capable at whatever he chose to do. In many ways it was never personal to him. After the war he assisted freedmen.
In the South there were agricultural organizations that discussed and promoted good animal husbandry generally, which included proper handling of disobedient field slaves (which were "all the same" after all).
Traders also had what today we call "best practices." One was to completely dominate ("break") your new charges into submission - making yourself their only source of survival. Another is that when you physically punish you don't leave marks. Human traders in the U.S. today use the exact same techniques.