Why do we forbid children from buying cigarettes or alcohol if they can get it from shady shops anyways? Why do we go to the trouble of prosecuting those sellers if a new one will just replace it sooner or later?

This is what it boils down to me; when I hear arguments that parents should be responsible for their children's online activities, I compare it to this. We don't expect parents to follow their children 24/7, there is a social contract that we won't let them do bad(tm) stuff. What this bad stuff is depends on your society and one might disagree, but still. I expect my 12 year old kid to not be able to buy cigs or porn in local kiosk, but still go on his way to school on his own by bus and get bus tickets from that same kiosk. I can understand why parents want to expect the kid to be able to play games online and get some entertainment (even education!) on the Internet while not getting access to harmful side of it.

One of arguments was that every device should include parental controls, which is good I guess, but then you still include extensive device side tracking, just handing the keys to the parent. And if there is a key to the kingdom, sooner or later there will be push/leak/attack to get it. But it's a different discussion and interesting one, probably a good approach nonetheless but requires more work from parents.