That's a good question, Woodchips.
I don't know what the law is Stateside, but over here, under the Designs, Patents and Copyrights act of 1988, the copyright is retained by the author for 70 years, unless specifically sold to another party. You can read it here if you like:
https://www.copyrightservice.co.uk/copy ... yright_law
Some of those mags listed are UK based, so I don't know how the authors are paid. It's easy to assume that they are not, but I don't know that to be the case. I do know that one of those publishers regularly infringes authors' copyright. They did mine, and I had to threaten court action to be taken seriously. We settled out of court. I know of several other authors who have been treated the same way by the same publisher.
It is, of course, a terrific service if the copyright of all the articles therein do indeed belong to the publishers rather than the authors, and those publishers have made them available for free. I don't see what is in it for them if they do, but if that is the case, well, excellent.
For those of us who rely on royalties and patronage for our work in order to put food on the table though, the infringement of our copyright, in those cases where is does occur, is a serious threat. In those cases it is theft and needs to be recognised as such.
I'm not saying that that has happened on the above site, I have no idea, but if it is all legit, I don't understand how it works and everyone gets paid.
I'm happy to be enlightened.