In the book the color purple why was Sofia arrested?

Sofia was in town when the Mayor's wife, Miss Millie asks Sofia if she would like to work for her and be her maid. Sofia is offended by this, just because she's black doesn't give anyone the right to ask her that. Sofia then says, "Hell no." which causes the Mayor to come over and ask Sofia what she said to his wife. Sofia is about to repeat it when he slaps her. Then Sofia punches the Mayor and the police come to arrest her.