How does Cassie reveal her innocence of the world on the day she goes to Strawberry

I think this is when Cassie reminds Mr. Barnett that he has been serving them at the store. A black person is not supposed to question a white person, especially in that way. Cassie is only doing what is fair and just. Unfortunately their world is anything but fair and just.

um well i read the book and tht is not the case.