> I don't know why, but spam on mailing lists seems relatively rare
Mailing lists have a central place where all messages come in, and that can ban spammers. Usenet is distributed and messages come from everywhere, so any spam protections needs to be done in the client.
Spam, AFAIK. I don't know why, but spam on mailing lists seems relatively rare, compared to usenet which was ~99% spam last time I looked.