People often miss the obstruction of justice at the end. Lee is brilliant in writing a novel about justice then sliding it past the reader as something we deem acceptable because of circumstance.
I feel like your view presupposes that the goal of the book, is that "justice should be the same, no matter who it's for". But (it's been a long time since I've read it) I feel like it's more about doing what is "right" than what is "legal".