[AnthonyMouse]

What is that copyright protection supposed to look like?

There are clearly things that combine function and expression. Essentially all computer programs do this. But if the expressive part can be copyrighted and the functional part can't then the combined thing has to be separated into what can be copyrighted and what cannot.

The thing about APIs is that by their nature they're purely functional. There is no expressive component that can be extracted and changed without changing the function itself. Even the name is functional because that string of characters is what the compiler uses to match invocations of the function with its definition.

Try a mirror image analogy. First, a popular lock manufacturer encodes the mechanical specification for its lock cylinder as music, and it turns out to have a pleasing sound. When the sound hits the top 40 they can have all the music royalties, but that doesn't mean they can copyright the lock itself as an expression of the music.

Second, a popular lock manufacturer starts selling locks which will only open if the key blanks are encoded with a particular sequence of musical notes composed by a human artist, and asserts that such key blanks are copyrighted. It doesn't matter how expressive the music is when you play it on a piano, or whether it would be copyrightable if it was reproduced as sheet music rather than key blanks. If you specify it as an interface then it can't be copyrighted in that context because the functional interface demands that specific expression.