why is "Shoot all the bluejays you want, if you can hit 'em but remember it's a sin to kill a mockingbird" relevant with repsect to the theme or symbolism in this book?

The Mockingbird

When Scout and Jem receive airguns for Christmas, Atticus tells them that although he would prefer that they practice their shooting with tin cans, if they must shoot at living things, they must never shoot at mockingbirds. Atticus explains that it is a sin to kill a mockingbird. Clearly, this is the title scene, but the theme continues throughout the book. Miss Maudie explains why Atticus is correct - mockingbirds never do anyone any harm, and are not pests in any way. All they do is sing beautifully and live peacefully. Therefore, it is a sin to kill them. The mockingbird comes to represent true goodness and purity. Tom Robinson is one example of a human "mockingbird". He stands accused of raping and beating Mayella Ewell, but is innocent of the charges. The town commits the ultimate sin by finding him guilty and sentencing him to death. In effect, they have killed a mockingbird. Boo Radley is another example of a human "mockingbird". He has spent his entire life as a prisoner of his own home because his father was overzealous in punishing him for a childhood mistake. Boo Radley observes the world around him, causing no harm to anyone, and then saves Jem and Scout's lives when Bob Ewell attacks. The sheriff determines that Ewell's death will be ruled an accident to avoid forcing Boo to go to trial, even though Boo killed him to protect the children. Atticus agrees, and wants to make sure Scout understands why this little white lie must be told. She replies saying of course she understands, putting Boo on trial and in the public sphere would be like killing a mockingbird. The mockingbird represents true goodness and innocence that should always be protected.