Why does Atticus reprimand Scout after her disclosure about going with Calpurnia to church?

First of all, Atticus does not reprimand Scout for going to Calpurnia's church in chapter 14. He only finds out that she and Jem had attended Cal's church after Scout tells him that it was then that she first asked for the definition of rape. Cal wouldn't tell her and referred her to her father. After her father gives her the definition of rape, and Scout doesn't understand it, she asks why Calpurnia wouldn't answer...

First of all, Atticus does not reprimand Scout for going to Calpurnia's church in chapter 14. He only finds out that she and Jem had attended Cal's church after Scout tells him that it was then that she first asked for the definition of rape. Cal wouldn't tell her and referred her to her father. After her father gives her the definition of rape, and Scout doesn't understand it, she asks why Calpurnia wouldn't answer her on the day she attended her church. At the time, too, Calpurnia had invited Scout and Jem to visit her home sometime and Scout asks Atticus if she can. Aunt Alexandra freaks out and interrupts by saying that Scout can not go visit Calpurnia. Scout turns her back on her Aunt and says, "I didn't ask you." This is what Atticus reprimands Scout for--disrespecting her Aunt Alexandra:

"For a big man, Atticus could get up and down from a chair faster than anyone I ever knew. He was on his feet. 'Apologize to your aunt,' he said. 'I didn't ask her I asked you--' Atticus turned his head and pinned me to the wall with his good eye. His voice was deadly: 'First, apologize to your aunt'" (136).

After the apology, Atticus makes sure that Scout understands that she must obey him, Calpurnia, and Aunt Alexandra. As a result, Scout hides in the bathroom until she feels like the coast is clear. Atticus wasn't angry that the kids went to Calpurnia's church; and, he wasn't against the kids visiting Calpurnia at her home, either. But Aunt Alexandra is so upset that she suggests that Atticus should fire Calpurnia because of it. Fortunately, Atticus doesn't see it that way and he does not fire her.