In To kill a Mockingbird by Harper Lee, why was Tom Robinson looked upon as the "mockingbird?"

In To Kill a Mockingbird, Atticus tells Scout and Jem never to shoot a mockingbird. When Scout asks Miss Maudie why, Miss Maudie says, “Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” In other words, mockingbirds are defenseless and loving creatures. They injure no one and only try to add to other people's enjoyment of life. Tom Robinson is a mockingbird-like creature because he is also defenseless. He does not injure Mayella Ewell and only wants to help her and pay her some attention because he senses that she is lonely. In addition, one of his arms was horribly mangled in a childhood accident, so he could not injure anyone. He is, however, wrongfully convicted of raping Mayella Ewell by a racist jury in Alabama in the 1930s. His wrongful conviction is akin to someone injuring a mockingbird because Tom is an innocent and defenseless man.