Atticus tells the childern they're not to kill mockingbirds because doing so is a sin. How does Miss Maudie explain Atticus' reasoning to Scout?

In Chapter 10 of "To Kill a Mockingbird,"Mis Maudie tells the children,

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.

Of course, this passage is significant in its symbolism.  Tom Robinson, the mockingbird of the town, is kind and innocent.  He does not bother anyone; he only helps the Ewell girl out of human charity.  Thus, the sin that the Ewells and the townspeople commit is in their distorted perception that Tom has crossed a civil line.  A man of his word and moral conscience, Atticus recognizes this sin and feels compelled to defend Tom because the moral rules supercede any man-made rule.