Most public libraries (you are in the USA?) have Fortune and most other magazines with lists absolutely free for use. I’m sure that you know that but a lot of people forget what a great resource their local library can be.
Indeed that is true, although I live in Ireland. My local library has the hard copies, and my university library has both hard copy and online subscription.
Why do you think it’s public information? Fortune Magazine puts together the list using information from corporations (which is public information). You’re perfectly welcome to get the relevant information from the Securities and Exchange Commission - or from every individual corporation in the U.S. - and put together your own list. But Fortune’s compilation belongs to Fortune.
It’s “public information” in the sense that Fortune is a magazine and publishes it. It is not “public information” in the sense that Achilles can’t just copy the list and call it the “Achilles 500”: the compilation is protected by copyright.
Fortune, and other similar compilations, don’t try to hide their lists (though they would be happy to charge you for it, if they can). The real value of such lists derives from their widespread use, unlike other, more proprietary lists that rely on the judgement of the compiler, like Dun and Bradstreet or Standard and Poors ratings.