Why is it a “sin” to kill a mockingbird?

It is a sin to kill a mockingbird because the mockingbird brings us nothing but joy.

“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.”