APIs are interfaces. If an interface is truly innovative in some important way, the proper way to protect it is with a patent, not with copyright. Of course, then you'll have to actually prove that it's sufficiently innovative - and, conversely, somebody could challenge your patent and strike it down by e.g. providing examples of prior art.
If all APIs are copyrighted, then interoperability goes out of the window. How far do you think would F/OSS have gone if it couldn't re-implement Unix enough to be reasonably compatible? The story of Linux vs Hurd tells volumes right there.
If all APIs are copyrighted, then interoperability goes out of the window. How far do you think would F/OSS have gone if it couldn't re-implement Unix enough to be reasonably compatible? The story of Linux vs Hurd tells volumes right there.