Filtering is necessary to protect children from evils on the Internet. The horror stories parents have heard about kids meeting strangers online are completely dwarfed by the horror stories that occur to children when they are not online. The Internet is one of the safest places a child can be. Children are safer sitting in front of a computer than they are in public. Parents don't need filters to protect their children online any more than they need a bodyguard to protect them in public.
Filters operate on principles of decency.
Filters are computer algorithms, rules which seek patterns and follow instructions based on what they find. Filters do not make judgments, act in loco parentis , evaluate context, appeal to morality or understand you or your child's needs. They simply scan for letter combinations. This constitutes neither the exercise of judgment nor the enforcement of decency. Decency, after all, is not a computable concept.