Teach people how to enable their own filters. Or would that be expecting too much personal responsibility?
I like to think it's because that would make too much sense.
Fact is there isn't enough accountability on the part of the parent. You hear horror stories from the States of kids as young as eight or ten shooting siblings or parents because they had their video games taken away. Everyone points to the video games themselves as a trigger for violence, and no one asks how the child came into possession of a gun in the first place. The whole thing stinks of irresponsibility, and no one wants to say anything about it, opting instead for the easier scapegoat.
A parent should be taking an active role in their children's technological development, be that with the internet, through video games or whatever. But the fact is, with this generation at least, it's far too easy for the parents to feign ignorance to avoid responsibility; claiming "I didn't know my child could access that sort of thing!" when it's in truth their responsibility to know. This broadband filter was being pushed as a stop-gap, something for the more irresponsible parents of last generation to fall back on.
My ultimate problem with it is that by next generation it'll be useless. The children that grew up with the internet will become parents themselves, and having that better understand of just what their kids could get up to on the net, will take an active interest in their browsing habits, making the widespread censoring of the internet a costly and wholly unnecessary step.