Hitting

I saw something about a girl that said when she was 15, she snuck a boy in and her mom caught her and her mom slapped her across the face. And the girl said that she wasn't mad at her mom, because it was her initial response to slap her. So, I don't understand. If there are two people in a relationship, and one of them hits the other, no one is gonna justify that as an "initial response." That's abuse. So tell me why it's okay for a mother to slap their child and it's considered a normal way of parenting?