The burden is on Google to show the APIs it copied do not contain copyrightable expression because Oracle registered the copyrighted works at issue, which include the API specifications and the code.
Sounds innocuous, and it's supposed to. But if accepted by the court, and on appeal, it completely transforms the software landscape.
Oracle is arguing that it can copyright APIs and prevent companies from even connecting to its programs with them, without its permission, which may include demands for payment or for other business arrangements, at its sole discretion.
This would be a profound expansion of copyright, which remember lasts for the life of the creator plus 75 years, and is routinely extended any times it seems like Mickey Mouse might enter the public domain. (There is constant talk of making copyright eternal as well, and eliminating the public domain entirely.)