In the novel To Kill a Mockingbird by Harper Lee, why does Miss Maudie say that it is a sin to kill a mockingbird?

In Chapter 10, Scout and Jem are shooting their air rifles and Atticus tells them that they can shoot all the bluejays they want, but it is a sin to kill a mockingbird. Scout mentions that it was the first time she ever heard her father say that it was a sin to do something and decides to ask Miss Maudie about it. Miss Maudie tells Scout that it is a sin to kill a mockingbird because they do nothing except make beautiful music for people to enjoy. She also comments that mockingbirds do not eat up people's gardens or nest in corncribs, and only "sing their hearts out for us" (Lee 119). Mockingbirds are essentially innocent beings that do not deserve to be shot or harmed. Throughout the novel, various characters, like Tom Robinson and Boo Radley, are represented as symbolic "mockingbirds" because they cause no harm to anyone. When Tom Robinson is wrongfully convicted of raping and assaulting Mayella Ewell, it is metaphorically comparable to a person killing a mockingbird. Tom Robinson was an innocent man who was unfairly harmed which demonstrates that Atticus' lesson can apply to respecting any innocent being, and not just mockingbirds.