Usenet is based on propogation. A person uploads a file to a news-server, this file then gets propogated to all news-servers connected to that one news server and so on. A new client (program on a downloaders PC) gets a list of all the files available on a news group and then selects the files to download, so in essence only the original uploader is the file sharer and the news server hosts for some reason aren't held accountable
So for P2P anyone downloading a file is then file sharing that file along with the original sharer, where in usenet only the original uploader is counted as the file sharer. Also as P2P you make a direct connection to the sharer thier IP is visible where in usenet the originators IP is only known by thier usenet provider...Hence the BPI are targetting PSP filesharers as it's simple to get thier IP