In Roll of Thunder, Hear My Cry how does Mama explain to Cassie what Mr. Simms did and how Big Ma handled the situation?

In Chapter 5 of Roll of Thunder, Hear My Cry, Mr. Simms forces Cassie to apologize to his daughter, Lillian Jean, for running into her. A rude man, Mr. Simms also pushes Cassie off the sidewalk. Cassie bumped into Lillian Jean accidentally when she was upset because Mr. Barnett would not wait on her in his store. Cassie does not want to apologize, but Big Ma forces her to do so. 

Later (in Chapter 6), Mama tells Cassie that Big Ma had no choice but to force Cassie to apologize to Lillian Jean. Mama says to Cassie, "in the world outside this house, things are not always as we would have them to be" (page numbers vary by edition). Mama also explains that Mr. Simms thinks Lillian Jean is better than Cassie because Lillian Jean is white and Cassie is African-American. Mama explains that racism has its origins in slavery. She says to Cassie:

"The people who needed slaves to work in their fields and the people who were making money bringing slaves from Africa preached that black people weren't really people like white people were, so slavery was all right" (page numbers vary by edition).

Mama explains that even though slavery has ended, people like Mr. Simms still cling to the false idea that African-Americans are inferior to white people.