Part of the problem is that pretty much everywhere such filters have been installed, sooner or later there's been revelations of perfectly legal content being blocked. More than once this has been political speech.

They're building all the tools of a police state. With the best of intentions, mind you. But it's not them we're afraid of (we might dislike having to opt in/out of porn, but that's merely a nuisance) - with some luck they'll get voted out in a few years time.

What at least some of us are afraid of is that some day down the line someone else gets into power who decides to take full advantage of all the nice tools their predecessors have given them. Put all the tools in place, and that someone might even be more inclined to seek power, knowing the power can extended all that much easier.