Alec started being physically abusive with Isabelle when she was young, so later on he can enact sadomasochistic kink in the guise of just being punishment. Isabelle doesn't inherently know that there's a difference between Alec beating her with his fists in the spur of the moment and Alec beating her with a riding crop in a more ritualized, intimate way. In her view, at that age anyway, it's all just pain that results from her being bad. She doesn't come to understand that Alec gets off on it until she's older.