"If it's posted to Usenet it's in the public domain." False.
Nothing modern and creative is in the public domain anymore unless the owner explicitly puts it in the public domain(*). Explicitly, as in you have a note from the author/owner saying, "I grant this to the public domain." Those exact words or words very much like them.
Some argue that posting to Usenet implicitly grants permission to everybody to copy the posting within fairly wide bounds, and others feel that Usenet is an automatic store and forward network where all the thousands of copies made are done at the command (rather than the consent) of the poster.
This is a matter of some debate, but even if the former is true (and in this writer's opinion we should all pray it isn't true) it simply would suggest posters are implicitly granting permissions "for the sort of copying one might expect when one posts to Usenet" and in no case is this a placement of material into the public domain.
It is important to remember that when it comes to the law, computers never make copies, only human beings make copies. Computers are given commands, not permission. Only people can be given permission. Furthermore it is very difficult for an implicit licence to supersede an explicitly stated licence that the copier was aware of.
Note that all this assumes the poster had the right to post the item in the first place. If the poster didn't, then all the copies are pirated, and no implied licence or theoretical reduction of the copyright can take place.
(*) Copyrights can expire after a long time, putting something into the public domain, and there are some fine points on this issue regarding older copyright law versions. However, none of this applies to material from the modern era, such as net postings.
Note that granting something to the public domain is a complete abandonment of all rights. You can't make something "PD for non-commercial use." If your work is PD, other people can even modify one byte and put their name on it.
