I think it's because people that abuse their spouses often eventually kill their spouse, or at least that is the perception, and so there is an inclination to be harsh with them right at the beginning. It might not be rational but I wouldn't be surprised if that was the source of it. There's probably an element of man beating woman worked in too, whereas random assaults are probably more man beats man and so there's less sympathy and outrage there, rightly or wrongly.
Good point regarding perception. There is the old saying... where you stand depends on where you sit.
This reminds me of the drug laws in the 1980's. Cocaine possesion had lower penalties because the perception was cocaine was for affluent white partying professionals who were a value to society. Crack posession was penalized more harshly because the perception crack was for uneducated blacks in ghettos who are violent and unemployed. 5 grams of crack triggered a minimum 5 year prison sentence versus 500 grams of cocaine.