I mean the courts aren't exactly robotic in this sense. It has to be a creative work, for the same reason you couldn't try to copyright a trivial sentence you wouldn't get copyright protection on a method description that adds two numbers. Also, it's the whole work that's copyrighted, not its individual parts. It's not that Google just borrowed a few method signatures, it's that they took all of them along with their relationships and meanings.
You aren't wrong, but the fear is that copyright trolls would immediatelly take massive advantage of this new opportunity as well as that it could have a chilling effects on open-source projects.
Even if eventually found to be in the right, a random guy with a GitHub repo is not going to have the resources/will to effectively defend himself.
