A public policy that is hard to understand grants legal migration status if an alien (usually a woman but sometimes a man) can argue successfully that the citizen or resident alien spouse was abusive.
Yes, society should provide short-term shelter for battered wives and issue court orders to prevent the repetition of the abuse; and it should probably extend medical care and counseling to the victims. But instant immigrant status for illegal aliens (or legal nonimmigrants) in this situation, regardless of numerical limitations?
This certainly is a nice benefit for the spouse in question, but is that the way we should distribute our limited supply of immigrant visas? What is the public good that is being provided?