You’re right that a faithful photograph of a public domain work is not copyrightable in the US. But consider this scenario. You run a website that allows volun

You’re right that a faithful photograph of a public domain work is not copyrightable in the US. But consider this scenario. You run a website that allows volunteers to upload books with covers derived from public domain artwork. Then you get a letter in the mail. You’re being sued for copyright infringement. The plaintiff claims one of your book covers is derived from not a 19th‐century classic work of art, but a 1957 painting made in a classic style. What do you do? A scan of a book page printed in 1928 that reproduces the painting would make the lawsuit go away very quickly. That’s handy. A webpage from a major museum with the painting clearly marked CC0 would provide evidence that you used the painting in good faith. That’s also handy. If you have neither one, all of a sudden proving this is actually a public domain painting becomes a lot harder. (Worst case scenario: it’s not, and you’re liable!) I’m not affiliated with Standard Ebooks, but I’ve always assumed this was the underlying reasoning behind the policy. It’s stricter than I would be if I ran such a site, but I can’t fault them considering the clear benefit it provides.