Why does society and people defend abusive parents?

They only punish and criticize parents who molest their kids and rarely condemn those who give their kids black eyes or call them names. Why is it OK for a battered woman or man to use self-defense against an abusive spouse but not OK for a son/daughter to defend themselves against abusive parents? We are often told to respect our parents no matter what and hitting them back is wrong. Whenever you hear about parents hitting their kids (minor or adult) people automatically assume the kid is at fault, they don't think for one minute that maybe the parent is the crazy one. I personally believe that the whole 'never argue with your parents' thing is bunch of BS when it comes to abusive parents. Sometimes you have to throw that out the window and not take any more harm just because they are your parents.