copyright has a fair use clause, which states that one can decompile software to produce a derivative for the purposes of interoperability. This then must assume that any apis being implemented has to both fall under copyright (otherwise, this law does not apply to api), but also means that the law allows for the use of the api for the purposes of interoperability (and therefore, you cannot claim to have the sole rights to an api).

Beware interpreting interoperability as a form of fair use. I wish we could make that assumption, but I don't think it's warranted.