According to cnet news:
One of the reasons Usenet was singled out in this lawsuit is because of some of their advertising language to potential subscribers: Usenet.com boasts that signing up for an account "gives you access to millions of MP3 files and also enables you to post your own files the same way and share them with the whole world."
In a lawsuit filed on October 12, the RIAA says that Usenet newsgroups contain "millions of copyrighted sound recordings" in violation of federal law.
Only Usenet.com is named as a defendant for now, but the same logic would let the RIAA sue hundreds of universities, Internet service providers, and other newsgroup archives. AT&T offers Usenet, as does Verizon, Stanford University and other companies including Giganews.
That's what makes this lawsuit important. If the RIAA can win against Usenet.com, other Usenet providers are at legal risk, too.
The future's not looking too bright for Usenet right now. If the RIAA somehow does win, then their slippery slope puts everyone at risk. The big winner in all this will be the legal system, who will have enough lawsuits to sustain several generations of lawyers and bureaucrats.