why did uncle jack spank scout?Why did Atticus belive she desered this punishment even after jack admitted his error

Uncle Jack spanked Scout because she punched Francis, her cousin, because he called her father, Atticus, a "Negro Lover." Atticus believed that she deserved this punishment even after Jack admitted he was wrong because she still punched her cousin. Atticus told her to stay out of fights and she didn't.

Uncle Jack spanked Scout after she had "split my knuckle to the bone on his (cousin Francis') front teeth." Scout never got along with her cousin Francis, and when he called Atticus a "nigger-lover," and then repeated the remark to her, Scout decided to take action. She waited for Franciks to separate himself from Aunt Alexandra, and then she let him have it. Jack pulled her off him, but when she tried to run, her uncle spanked her, leaving "seven or eight red marks."

Atticus wasn't too upset. He chuckled. "She earned it, so don't feel too remorseful," he told his brother.