I didn't say all text was copyrightable; I said anything that isn't text -- or any other fixed form -- isn't. A REST protocol (i.e. a REST API) doesn't have a fixed form. You can describe the very same protocol using many different texts (i.e. documentation) -- something which you cannot do for an actual API, which is text. While each of those documentation texts may be copyrightable in itself (technical documentation is very much copyrightable), the REST protocol itself isn't. It is only a mechanism, whereas an API is indeed a mechanism, but it is also a text. Since it is a text, it might be copyrightable, and since a REST protocol isn't a fixed text -- it is certainly not.