I don’t think that kind of thing is unique to the South nor its link to slavery. In a larger scope, it’s a deference to class hierarchy. George Orwell in Homage to Catalonia, talking about his experience in socialist Barcelona during the Spanish Civil War:
Waiters and shop-walkers looked you in the face and treated you as an equal. Servile and even ceremonial forms of speech had temporarily disappeared. Nobody said ‘Señor’ or ‘Don’ or even ‘Usted’; everyone called everyone else ‘Comrade’ and ‘Thou’, and said ‘Salud!’ instead of ‘Buenos días’. Tipping was forbidden by law; almost my first experience was receiving a lecture from a hotel manager for trying to tip a lift-boy.