That parents' behavior has no effect on their children is not a universal belief. I think that a belief that one bears no blame for what one's children do is a way to inevitably raise terrible children. That's like not having any responsibility for whether the product you built works, or is toxic.

edit: blame isn't exclusive; everybody can have 100% of it. Although IMO blaming children for what they've done (however you define the age of majority) is almost entirely scapegoating. You might as well convict a dog or a pig as an adult if you would convict a 13 year old.