Except it is the law now and then that APIs aren't copyrightable according to the court that's generally supposed to handle such things, in this instance The US Court of Appeals for the Ninth Circuit. They've ruled plenty of times on the matter, and have developed decades of case law stating that implementing APIs is totally kosher. Sony v. Bleem is great example. There the appeals court ruled that not only could Bleem ship a PS1 emulator with reverse engineered APIs of the kernel (explicitly as a commercial venture even!), but they could port it to a competitor's game console (the Dreamcast), and use Sony's IP in screenshots.

But, because Oracle v. Google case had an ancillary patent question in an earlier appeal, the appeals are instead going to the Court of Appeals for the Federal Circuit. This court is really only supposed to answer patent questions. The CAFC can't set precedent in these matters, and overwhelmingly defers to the district courts' precedent. Bot for some reason they have chose to ignore all of that this time and make their own, fresh (and super weak) precedent that only applies to copyright cases with patent questions.