You are right there is no single national case that clearly ruled in either way, but the status quo is that it's de facto ok. Adjacent case law made white room reverse engineering & API re-implementation "ok" de jure, which is why most storage vendors - including very large ones - are confident enough to implement the S3 protocol without securing a license from Amazon first.
Edit: none of the large companies (except Oracle) are foolish enough to pursue a rule that declares APIs as falling under copyright because they all do it. In Google v. Oracle, Microsoft files briefs support both sides after seemingly changing their mind. In lower courts, they submitted an amicus brief supporting Oracle, then when it got to SCOTUS, they filed one supporting Google, stating how disastrous it would be to the entire industry.
Edit: none of the large companies (except Oracle) are foolish enough to pursue a rule that declares APIs as falling under copyright because they all do it. In Google v. Oracle, Microsoft files briefs support both sides after seemingly changing their mind. In lower courts, they submitted an amicus brief supporting Oracle, then when it got to SCOTUS, they filed one supporting Google, stating how disastrous it would be to the entire industry.