They don't, exactly. They are responding to the signals he is sneding that the man will defend her and her children with the same ferocity that he uses to abuse them. Her children will thus have a better chance to survive to adulthood with a strong, powerful male around - what happens to her in the interim doesn't matter. The nice guy, on the other hand, is sending signals that he will be a pushover, and will not be strong enough to defend the family in a crisis.
Note that this is all happening at the subconscious level, and I doubt one in a hundred women could explain to you why they act the way they do toward abusive men. But usually only a woman with a strong sense of who she is is able to overcome the genetic programming.
And be a shopaholic b*tch....