The proponents of this filter are computer illiterate 50+ year old people, the conservative base. These are exactly the people that will not know about switching the filter off. Their children will get around the filters, but these dust-covered voters will believe that the internet is a little bit more cleaned up, and a lot less useful, than it really is. If you block porn you need to block user generated content, which excludes the vast majority of blogging and web 2.0 platforms.
Self-imposed irrelevancy for the computer illiterate. We'll block ourselves from being able to access these new scary things, and then pretend like we've solved a problem.