Right, but nobody is asserting an implied license claim (as far as I'm aware). The complaint is whether or not any API declaration bears copyright, and if so, whether or not you can make a fair use of such a declaration. They aren't arguing whether or not publishing said declarations implies copyright permission to use them. Granted, that would probably give the Supreme Court an easy out on this decision without having to throw out a lot of caselaw that seems to hint towards API copyright or suddenly reassign ownership of half the world's operating systems code to a handful of defunct holding companies.
API is mechanism, honestly I do not understand all that fuss.
Another API implementation is compatibility layer for authors who heavily invested in the platform. They are the reason platform is successful. Car dealerships investment is protected.
Same could be applied to any public interface. Header files, system calls, standard library, hardware version, language version, network interface.
Linux and WINE replaces Windows entirely. Looks like a lot of fans wait exactly that, not ReactOS.