I see your point, but I think you are falling into the trap of the people who want feminist characters in Tolkien. It is like saying that any book that does not contain strong tractordriving women is contra-revolutionary and anti-feminist.

The book is about a miscarriage of justice, and it is narrated through the point of view of a white, female child in the South. It is not terribly surprising that this child is not in contact with as wide a selection of black society as she is with white: her access to it it only through Calpurnia's church and her father. Introducing a second court case would have made the thing messy, and confused the issue, which is the miscarriage of justice based in racism; not racism as a whole.