The filters are supposed to block access to sites parents wouldn't want their children to see. Now that the Supreme Court has ruled the CDA unconstitutional, that bluff has been called.
Software companies have two different approaches to marketing the filter programs.
One evokes law enforcement. It provides parents with a suggested list of sites to exclude, but nothing is excluded without being specifically designated by the parents. All the filter products claim to have sophisticated algorithms to avoid these pitfalls. The challenge is in applying a formulaic approach to a heavily context-sensitive sorting problem. As Devlin's example shows, this is easier in theory than in practice.