Indeed, an answer of "no, they're not copyrightable" would leave the world generally how it is today. An answer of "yes, all existing APIs are copyrightable" would be tremendously impactful in all sorts of ways I can't even imagine. Presumably someone would immediately sue somebody else because of a tenuous claim to ownership of, say, HTTP or some JavaScript extension.
Microsoft has filed an amibus brief for the "not copyrightable side," as has the EFF, IBM, Red Hat, and a team of 83 computer scientists.
You should probably note the folks on the "yes, copyrightable" side for future reference as well, including Dolby, the Motion Picture Alliance, SAS, the DoJ, the Recording Industry Association of America, and also 4 CS professors (Dr Spafford of Purdue, Dr. Ding of UC Davis, Dr Hollaar at Utah H, and Dr. Porter at maryland U).
This thread is about web or network service APIs, which, thanks to the CFAA, have broad leeway to dictate what client software you are legally allowed to use to speak to it. It's a grey area and some real bullshit, IMO.
You are talking about programmatic APIs, which is a horse of a different color: a copyright issue, which is still being figured out.
It's annoying that we overload the same term for both things.
Referring to the other side in a debate as "the devil," aside from being hyperbolic to the point of inducing an eye roll in every reasonable person within earshot, is exactly how the US ended up with a reality TV show host in the White House.