Ironically, it encourages good manners among the upper classes, because they are taught to distance themselves from the lower classes through particular behaviours proper to feudal landowners.
People somewhat lower down also adopt these manners, hoping to be accepted higher up.
A Different Perspective
Slavery wreaks havoc with "manners" because it divides humanity and allows people of means to treat other people as less than human. Their children grow up under that artificial construct thinking it is permissible to hurt other people, think of them as inferior, treat them as possessions, take away their rights and treat them as animals. Slavery promotes savage and ill-mannered behavior by people who should know better and the very concept of "manners" loses all meaning by being swamped in hypocrisy.
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Slavery
The Harpers Ferry raid showed how close the nation was to going to war over slavery.
Armed conflict
There are a number of things that Abraham Lincoln did for the nation. He abolished slavery and was the one who proclaimed Thanksgiving as a holiday among other achievements.
For the same reason the US justified slavery.