There is no human moderator who decides which e-mail to send to the group. People who participate in such groups generally regulate their behavior voluntarily. Bulletin board systems and conferences and newsgroups are different ways of organizing public group conversations where nobody is the moderator or editor. There are moderated groups where an expert in the field acts as editor, deciding which of the submissions are published. If the Communications Decency Act is enforced, all unmoderated sites will either have to go out of business or set up pre-screening to make sure only adults get access.
Most unmoderated sites are non-profit.
They have a volume of both participants and of messages that is too large and too widespread to permit prescreening. In addition, many unmoderated sites have been set up long ago and the person who set them up is no longer involved. Thus, there is no one around to do the screening. For these reasons, many of the sites would have to be totally eliminated. I fear that moderated groups won't fare much better.