Since there are public access news servers that carry these news groups, and pornographic FTP sights, it is trivial for users to circumvent these restrictions. Also, there are multiple paths to the same information. In order for a system to make it impossible to get to unhealthy or illegal information, it must limit the user's access to a well defined set of sanitized information. In many cases, this is unacceptable. The problem of jurisdiction is also serious.
Obscenity law depends on community standards .
However, the Internet and other large networks span many communities and are influenced by their community standards. However, the sysops were prosecuted under Tennessee law because their material was accessible there. Soon after a guilty verdict was announced, there was discussion of what this meant for Internet sights. Within Chicago what is on the net wouldn't violate community standards, especially given what I can rent at the local video store . . . The problem of jurisdiction is minor in the US, when compared to other countries.