Why does Aunt Alexandria accept that the Cunninghams may be good but are not " you type of folks"?

In Chapter Twenty-three, when Scout wants to invite Walter Cunningham over for lunch more often, but Aunt Alexandra puts her foot down, saying that the Cunninghams aren't the right sort of people for Scout to spend time with. Scout can be gracious to Walter and polite, but can't invite him over because "he is trash."

Class divisions aren't beneficial to anyone..... everyone has something to offer.