Because out of everyone, Kanon was most likely the only one who wasn't cruel to her. She is only giving him the same courtesy by not leaving immediately.
ML didn't even know how the FL was being treated and he is going off based what the assistant said and what she said and came to his own conclusion that she had enough and finally grew a spine. Of course, not knowing and being a way from the house doesn't excuse much of anything but not like he had anything to do with giving the order to treat his wife like she was nothing. The servants acted in their own and got punished accordingly by her after receiving the permission to do so as it's her duty as the Lady of the House.
Everyone else who were cruel to the Abused Ram can die though.
Bruh .. i dont kno why she suddenly so attached to the son... Once she got her memory back she should just announce her divorce and just get away from there... Why should she consider any of their feeling? I thought shes the greatest witch or whatever? Why did she just let herself be contain in that house? She turned into typical shoujo manga girl and here i thought shes gonna be a badass