In some (most?) cases the partner of the abusive caregiver absolutely does know about the abuse, and conceals, makes excuses or even participates to some degree. As is typical in abusive family situations of any sort, there can be a lot of self-deception involved, or victim blaming, or just plain refusal to see the signs. Often if they extricate themselves from the situation they'll pair up with another abuser, because whatever's broken in them that attracts abusers will still be broken. It's amazing the number of kids who wind up right back in foster care, and it's not fair to the kids.
For that matter, I think you're unfairly characterizing the situation, as I've never heard of a CPS agency anywhere that declares DV victims irredeemably unfit. Heck, it takes years to get CPS to declare physical abusers unfit (or incorrigible drug addicts, molesters, people who can't stay out of jail for long enough to make a sandwich) and stop trying to reconcile their families.
It's important to be on the look out for dysfunction on the non-abusive parent if that dysfunction is part of the abusive situation.
PS There is no "acceptable" level of hitting children.
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