Many (or even most) so-called "public" servers are not intentionally "public". It is fairly common for an inexperienced news server administrator to inadvertently configure his/her server so that outsiders can connect to it. Sooner or later he/she is bound to notice it, and will probably shut off outside access at that time.

If you get a "no permission to talk" or "no DNS entry" error, try again later. This usually means the machine is too busy to log you in, not that you don't have permission to enter later. These sites will usually reappear and become accessible again in a few days, as soon as the hits on their system slow down to a manageable level