Why does Scout get into trouble with Calpurnia and have to finish her lunch in the kitchen?

In chapter 3, Walter Cunningham Jr. eats dinner with the Finch family and politely asks Atticus to pass him the syrup. After Calpurnia returns from the kitchen with some syrup, Walter Jr. proceeds to pour it all over his vegetables and meat, which disgusts Scout. Scout cannot control herself and rudely asks what the "Sam Hill" Walter is doing. Walter Jr. gets embarrassed, and Scout continues to draw attention to his odd table manners. Calpurnia then...

In chapter 3, Walter Cunningham Jr. eats dinner with the Finch family and politely asks Atticus to pass him the syrup. After Calpurnia returns from the kitchen with some syrup, Walter Jr. proceeds to pour it all over his vegetables and meat, which disgusts Scout. Scout cannot control herself and rudely asks what the "Sam Hill" Walter is doing. Walter Jr. gets embarrassed, and Scout continues to draw attention to his odd table manners. Calpurnia then takes Scout into the kitchen and chastises Scout for her rude behavior.

Calpurnia tells Scout that she should never contradict anybody on their eating habits and should consider Walter Cunningham Jr. their guest. Calpurnia then chastises Scout for acting so "so high and mighty" and says that it does not matter if Scout's family is considered better than the Cunninghams. Calpurnia demands that Scout show Walter Jr. respect and tells her,

If you can’t act fit to eat at the table you can just set here and eat in the kitchen! (Lee, 25).