Trying to get up to speed on this whole issue .. but as a software engineer, I'm a little surprised APIs are not copyrightable. Putting aside the pedantically literal creative work that goes into documenting an API and naming functions, the chief engineering role of the API is,

- breaking the domain of work into well modeled types

- information hiding

- keeping dependencies together and/or variously keeping data dependencies apart

- leaving room for extensibility

- speed of operations

- and promoting the correct use i.e. single v. multi-threaded or one server vs. distributed

That's real work, and getting a "good API" i.e. recognized throughout industry as well adapted to the problem domain (say SQL for DB work) or copy-on-write for C++-strings almost always reflects a better implementation which bubbles into the API.