They do block sites they say they won't. It is not possible for software or humans to properly monitor or label Internet content. The more than 1.5 billion Web pages on the World Wide Web change constantly, and thousands of sites and millions of pages are added every day. Most filtering software vendors won't reveal their criteria for blocking sites - some of which go far beyond the merely offensive and into political, religious or ideological reasons. Most don't provide lists of sites that are blocked, so users have no way of knowing what useful content they are being denied.
Filters are expensive to install and maintain.
But that is a paltry amount compared to the taxpayer dollars required to defend against lawsuits challenging the constitutionality of the filtering policy. There is no way to measure the cost of degrading the potential of the Internet as a learning tool. Despite all that, the filtering forces have a good chance of prevailing when they target the local library. Because those who oppose censorship rely on good hearts and common sense to work such things out. They fail to recognize the depth of guilt felt by parents who yearn for technology to help them guide and protect their children.