Before we go any further - a brief definition of APIs "An application programming interface (API) is a specification intended to be used as an interface by software components to communicate with each other. An API may include specifications for routines, data, structures, object classes. In 2010, Oracle bought Sun Microsystems, which developed Java. When it implemented the Android OS, Google wrote its own version of Java -but in order to allow developers to write their own programs for Android, Google relied on Java’s APIs."
This is what the EFF have to say:
"Here’s the problem: Treating APIs as copyrightable would have a profound negative impact on interoperability, and, therefore, innovation. APIs are ubiquitous and fundamental to all kinds of program development. It is safe to say that all software developers use APIs to make their software work with other software. For example, the developers of an application like Firefox use APIs to make their application work with various OSes by asking the OS to do things like make network connections, open files, and display windows on the screen. Allowing a party to assert control over APIs means that a party can determine who can make compatible and interoperable software, an idea that is anathema to those who create the software we rely on everyday. Put clearly, the developer of a platform should not be able to control add-on software development for that platform.
Take, for example, a free and open source project like Samba, which runs the shared folders and network drives in millions of organizations. If Samba could be held to have infringed the Microsoft’s copyright in its SMB protocol and API, with which it inter-operates, it could find itself on the hook for astronomical damages or facing an injunction requiring that it stop providing its API and related services, leaving users to fend for themselves.
Another example is the AOL instant messaging program, which used a proprietary API. AOL tried to prevent people from making alternative IM programs that could speak to AOL's users. Despite that, others successfully built their own implementations of the API from the client's side. If copyright had given AOL a weapon to prevent interoperability by its competitors, the outcome for the public would have been unfortunate.
Setting aside the practical consequences, there’s a perfectly good legal reason not to treat APIs as copyrightable material: they are purely functional. The law is already clear that copyright cannot cover programming languages, which are merely mediums for creation (instead, copyright may potentially cover what one creatively writes in that language). Indeed, the European Court of Justice came to just that decision last week (Ironically enough, when Sun Microsystems was an independent company, one of its lawyers wrote amicus briefs arguing that interoperability concerns should limit copyright protection for computer programs.)
Improvidently granting copyright protection to functional APIs would allow companies to dangerously hold up important interoperability functionality that developers and users rely on everyday. Let’s hope the judge agrees."https://www.eff.org/deeplinks/2012/05/oracle-v-google-and-dangerous-implications-treating-apis-copyrightable
SAS Institute Inc. v World Programming Limited : CJEU: "to accept that the functionality of a computer program can be protected by copyright would amount to making it possible to monopolise ideas, to the detriment of technological progress and industrial development".