This is a very rare incident in the Book of Mormon, not because of its content so much as its recording. It is one of the few times that we learn how it is that someone knew of the happenings in other locations. While we cannot tell specifically why Mormon would tell us this tidbit of information and not others, we might suspect that it is because it put Morianton in a bad light, and Mormon wants to paint him as a bad. Mormon prefers to paint this characters with rather distinct brushes. He wants the good to be very good, such as Moroni, and the bad to be very bad. Thus we have Morianton the wife-beater and earlier we had Mormon’s treatment of king Noah, which appears to have ignored the good that Noah did in building his kingdom.
What we also learn is that Mormon does not condone violence towards women. Since he includes this episode to indicate the depravity of Morianton’s moral character, we may safely assume that this act of physical violence is one that was not condoned. Certainly that remains true today, and the entire syndrome has had enough publicity that we know it now as abuse. Abuse was not condoned in Mormon’s time. It is not condoned now. The Lord has never condoned it.