Why, in many states, domestic violence requires somebody be arrested

when there is a call to police.

I am not an expert in domestic violence, nor affiliated with any organizations focused on domestic violence.  I am lucky enough not to be a domestic violence survivor, and ethical and empathetic enough not to be a perpetrator.  

What I am is a person who, having done my Feminism 101 on my own time, has learned a bit of attention to domestic violence over the years.

Here's the deal: people who are being subjected to domestic violence, usually but not always women (and not always in opposite-sex relationships) are not people with otherwise healthy relationships whose partners suddenly hit them one day.