Requires little or no effort to maintain. Automated decision-making ensures complete objectivity. Eli the Bearded, the moderator of alt.sex.stories, is willing to share his moderation bot expertise. Automated decision-making ensures completely objective incompetance. Difficulty in getting news servers to accept moderation of already existing group.
Spammers can and do forge approvals.
Servers which do not accept the moderation will continue to propagate spam. Propagation of messages outward tied to site where moderation is done. Possibility mistake made by moderation bot will cause on-topic posting to be accidentally discarded, or spam allowed through. Would prefer creation of new group to moderation of existing one, but will probably grudgingly concede. Remaining messages would then be posted.