Other than just wanting to be able to copy other people's work as they often do all the time anyway, I'm not sure I understand major tech giants' argument as to why APIs would not be copyrightable. They are, if anything, much harder to make well than their implementations.
I think copyright is appropriate because then people can make similar APIs but must demonstrate they were creating their own thing, not cloning. That's copyright. Patent would exclude anything doing basically the same thing, which seems very bad.