why does the incident with the powhitetrash children in the store distress and humiliate marguerite?

Even though Momma Henderson owns the land they live on, she cannot reprimand white girls for their rudeness or disorder; such is the influence of racial codes, that allow privileges to whites just for being white, even when these privileges are abused.

The differences between the old codes of obedient behavior that black people had to observe, and the frustration of young people with these codes, is shown through the juxtaposition of Momma's and Maya's reactions to the young girls who come to the store. But, after Momma's "triumph" over the girls, it seems as though people can be more dignified even if they follow these outdated and insulting codes. Young Maya's anger is justified and understandable, but is anger the best way to fight ignorance in this case? That social codes are being questioned, and perhaps cast aside, is an important theme in the book; this questioning will change race relations, and inevitably lead to friction.