I'm no legal scholar, but I think I see the distinction. By ex post facto, the US Constitution refers to a law that allows prosecution of offenses that occurre

I'm no legal scholar, but I think I see the distinction. By ex post facto, the US Constitution refers to a law that allows prosecution of offenses that occurred before the law was passed. That allows for all sorts of horrible abuses of power. I think it's safe to assume that there's a grandfather clause in the US law to comply with the Berne Convention, in order to allow uses of works under copyright that were in public domain at the time. If that's the case, then it wouldn't be considered ex post facto according to the US Constitution. Going forward, however, the work is copyrighted and can't be used without the permission of the author.