I disagree. Whether or not it is in the interest of the vendor is irrelevant; if APIs are copyrightable, the vendor's license extends to any work that uses that

I disagree. Whether or not it is in the interest of the vendor is irrelevant; if APIs are copyrightable, the vendor's license extends to any work that uses that API. If they are not, the vendor has no say in who can reimplement or use that API, as they have no ownership rights.