If your child steals a car, are you allowed to say "I can't watch my kids all time time" and get off consequence free? Of course not. Do I think it is realistic for parents to keep an eye on their kid 100% of the time, of course not.
I don't understand how you reconcile these 2 statements. They can't watch their kids 100% but also if the kid does something illegal, it's their responsibility? You can raise your kids to do whatever you want but they often do not listen.
If they aren't ready for unsupervised access to the internet, then you shouldn't allow it.
Good luck telling your kids they can't get on the internet. Remember you can't watch them all day.
Point is, if the parents are irresponsible, then who does it hurt? The parents? No. It hurts the children, and it hurts the rest of society.
Your same argument can be used to abolish any sort of legal protections for children like gambling, driving, smoking, drinking, tattoos, etc. Everything should be the parents' responsibility, right?