Yeah, the crux of the issue would definitely be whether use of an API is prohibited by default under copyright law for a country (i.e. does using a library make something a derivative work of the library). In the US, at least, the Google v Oracle case makes me think this is worst case fair use (for many contexts) and best case too functional to be covered by copyright in the first place.

Though I can certainly imagine that a multinational company might not be confident of the copyright status of API usage in all countries they operate in.