Someone left the barn door open. In this case, someone has set up a Usenet NNTP server, is getting feeds of articles from a peer or two, but has forgotten to put a password on the server. So anyone who finds the address can just connect and download articles. While this does happen from time to time, it's pretty rare these days. Anyone with a fat enough pipe and enough disk space to host a newsgroup server of any size also has some decent knowledge about what they're doing, and leaving the barn door open is a mistake they are unlikely to make. Further, as soon as an address is found and people start to connect, the host's bandwidth gets so overwhelmed that they fix their mistake pretty darned quickly. Anyone who isn't overwhelmed in a few hours probably doesn't have any newsgroups of interest (i.e. binary newsgroups). Further, there is some question regarding the legality of connecting to a newsgroup server without their permission and taking advantage of their mistake. Typically it's easier for the host to just close the door and forget about those who sucked up their bandwidth for a while, but it is not inconceivable that a law firm somewhere would be willing to take on a damages case against an uninvited reader. In a nutshell, open barn doors are rare, don't last long, and typically don't have any interesting groups.
Face it, if you want the good stuff, you're gonna end up paying for it. Still, a good usenet service is much cheaper than a web site subscription and looks better on the credit card bill.