As I undestand it, there is a principle that legal contracts themselves cannot be copyrighted, despite the fact that they might well be the result of a consider

As I undestand it, there is a principle that legal contracts themselves cannot be copyrighted, despite the fact that they might well be the result of a considerable amount of creative work. The underlying principle here is that the legal effect itself cannot be subject to exclusive ownership, and the only way to get the same legal effect is to have the same contract. I believe APIs can be considered under an analogous principle - that is, the ability to interoperate with a set of existing code shouldn't be subject to exclusive ownership, and you can't interoperate with the same existing code without implementing the same API, so APIs should not be copyrightable. (This draws a distinction between the API itself and the underlying implementation of that API).