Right, what is government is doing when a sexual predator has to register and not live near a school? Isn't it taking care of the children of the community. Public Policy is more complicated than bumper sticker slogans.
First, there a major difference between restrictions placed on convicted criminals, and restrictions placed on every citizen of an entire country.
Second, one represents a danger to the physical safety of others, and the other represents something that you would prefer your child not to see - do you not see a stark difference between a child being kidnapped and raped, and a child accidentally clicking onto a pornographic website?
Its not that complicated - we punish those who have committed crimes, we do not do so preemptively to entire populations.