Despite the fact that Walter comes from a poor family, Atticus treats him like an equal. In Chapter 3, when Scout watches Walter, perhaps because he doesn't often get a lot to eat, and the way he eats, she makes fun of him. Calpurnia pulls her aside and says, "Don’t matter who they are, anybody sets foot in this house’s yo‘ comp’ny, and don’t you let me catch you remarkin’ on their ways like you was so high and mighty! Yo‘ folks might be better’n the Cunninghams but it don’t count for nothin’ the way you’re disgracin‘ ’em—if you can’t act fit to eat at the table you can just set here and eat in the kitchen!”
Scout and Jem are being raised in an environment where they are taught to consider the perspective of others and to be courteous to people no matter what social class they come from. Atticus recognizes the social traditions and the unfortunate way those with more money condescendingly treat those with less, not to mention the racial bigotry that is still prevalent in Maycomb. In fact, he and Walter Cunningham Sr. arrange for a barter payment for Atticus' service as a lawyer because Atticus knows Walter Sr. has no other way to pay him.
This was set during the Great Depression. More than anyone in the novel, Atticus recognizes the financial struggles most people are going through. In any case, he treats everyone the same, regardless of income level.