In this chapter, Atticus tells his children that"it's a sin the kill a mockingbird". What reason does he give for saying this?

From the text:

Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird."

That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.

"Your father’s right," she said. "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."