Here is how newsgroups work:
Let's say I post a message to alt.fan.starwars, which is a discussion group about Star Wars. The copy of the message I post goes to my local server, which is part of a huge network of servers, and each server copies everything that goes through it to all the other servers it "talks to," until every server has a copy. Now, with discussion newsgroups, this is simple, because those discussions happen in plain text, which takes up very little bandwidth or hard drive space. Because of this, access to text-only newsgroups (such as alt.fan.starwars), can easily be had for free. In fact, Google even lets you browse text-only newsgroups. But you can't download music or videos or anything using Google's free access. But that's just for discussions.
Without the downloading, newsgroups are basically the original message boards (they really aren't that much different from OriginalTrilogy.com). In fact, if OT.com had been started fifteen years ago, it would likely have been a newsgroup. The one main difference is that OT.com requires people to sign up for an account, while newsgroups are really like public bulletin boards. Anyone can post a message that anyone can read and anyone can respond to.
With content like music, videos, audiobooks and similar "stuff," on the other hand, things work a little bit differently. The newsgroup alt.bin.starwars is a binaries newsgroup (the "bin" in the name stands for binaries), so you won't find any discussion at all, only movies and soundtracks and poster and comic books scans and audiobooks and stuff like that. THOSE kind of files take up huge amounts of room, so it's typical for most local servers to delete those files after only a few days, if not sooner. This quick deleting would make newsgroups lousy places to share content, since the content would be gone almost immediately.
However, there are companies (such as the companies I mentioned, Astraweb and GigaNews, along with others), who make copies of every single bit and byte of information that is posted, and store it on their own servers. The big draw for people to use those companies is because they have incredibly long retention rates. As an example, Astraweb now offers 742 days of retention, and GigaNews is advertising 752 days of retention, so in both cases, something posted today will still be available from them two years from now. In the case of your Salvage editions, someone going as "Bullzeye" posted them 480 days ago, and someone going by "CBA" (your moniker but backwards?) posted them more than two months ago. And they'll be there for a while.
When you pay for Usenet access, what you're basically paying for is the convenience of gaining access... not access to any specific content... but access to the content that they have backed up and stored on their servers in order to make it available to you long after it would otherwise have been deleted. And yes, if someone signs up only to get a copy of your work, then that's kind of the same thing, but that's not really how they operate. They just charge per Gigabyte of download, no matter what you're downloading.
So, to answer your question about why someone should pay in order to download from alt.bin.starwars, I guess it's because paying for newsgroup access really is the only option. If there were no groups like Astraweb or GigaNews, the content would be deleted almost immediately, in order to make room for more content. There are probably still some Internet providers who provide free access to newsgroups, but most have cut off access. Mainly this is because there is so much "stuff" on it that not only would it require a huge amount of bandwidth, but they'd probably get sued for copyright infringement.
If you're not familiar with usenet/newsgroups, what I was suggesting probably sounded pretty off-the-wall, because you're right, alt.bin.starwars doesn't charge anything, but really, that's just how newsgroups work. Ask around to people who use the newsgroups to transfer files back and forth. In browsing through alt.bin.starwars, you'll actually see a lot of fan edits (some of them labeled "beta," because I assume one person working on an edit is using the newsgroups to send it to someone else working on the same projects), while other people are sharing different audio tracks (again, likely to be used in fan edits). I can almost guarantee that those people have either purchased an account with one of the companies I mentioned or are lucky enough to have a school account where newsgroups access hasn't been shut off yet. :-)
Hope that helps explain how newsgroups work!