I really just don't understand it. I think that all areas of the USA have manners - some good and some bad. I think that all areas show their manner - again, both good and bad - in different ways and that people like certain ways over others. I have family in Ohio and they don't have the same types of manners that we do, but the intent behind what they do and what we do is all the same. My roommate in the College Program was from Wisconsin and she was as friendly as they came. I have yet to figure out what slavery has to do with how we show our manners in current times, so I am assuming that was a giant troll.
Not to let this degenerate into a regional slugfest, but I'm sure that the reference had to do with reminding us that conspicious displays of "good manners" don't necessarily mean that one is a moral and upright person; there is always the possibility that a person can hide evil behind a facade of social respectibility. Slave-owning isn't the only example of this, of course; domestic violence also can happen in very well-mannered families. Good manners are just that and nothing else, evidence that one knows how to navigate social waters. They are not really a reliable indicator of the purity of one's character, and that holds true no matter where or when you were raised.
History does have a lot of bearing on how we show our manners in current times, because the social niceties that we are taught in childhood are usually passed down through families over generations. There are cultural spins as well; my own parents were immigrants from a country where social class was still very much a permanent thing when they were raised: no matter how much money you ever made, your class remained the one you were born into, and the manners that you were taught reflected that. They passed on those things to me, and I've had some issues because of it. One example is that I was taught at home that I was never to make eye contact with a work superior or a teacher, because in their culture that was disrespectful. I was in my late 20's before any supervisor ever thought to directly ask me about it, and I was surprised to find out that Americans viewed it completely differently -- suddenly, puzzling incidents from my younger days had an explanation.
I grew up in the Deep South, and I do tend to follow Southern social conventions for the most part; they are ingrained in me, and I tend to try to pass them along to my own children. However, they don't often see those conventions used by the people around them where we now live, and so they don't come as naturally to them as they did to me when I was the same age. FWIW, I still call all strangers and all authority figures Sir and Ma'am, regardless of age; I figure that when in doubt, it is better to err on the side of too much deference rather than too little.
PS: Here is another "bratty child who has been drilled in polite address" funny story for you. A few years back, one of the stricter teachers in the parochial grade school that DS attended at the time was a victim of vandalism: his car was keyed on multiple occasions, and once a message was even gouged into the paint. He was (understandably) very upset by this, and it led to his eventual resignation from the school. However, the police officer who responded to the message incident ended up telling the Principal that he was impressed by how much the school emphasized proper forms of address: it seems that the vandal who carved the message wrote, "Mr. X is an a***ole!" (Naturally, this also told the cop that the person who vandalized the car had to have been a student at the school -- no one else would have written the insult that way.)