You can't copyright RESTful APIs as a class, just like you can't copyright leather-bound books as a class. You can only copyright a specific expression of an idea or collection of ideas. The novel printed inside the leatherbound book is copyrightable, and I would argue strongly that Basecamp's API is also copyrightable and should be copyrightable.
It was possible at one point to patent RESTful APIs, but no longer.
It's not clear to me how declaring APIs copyrightable impacts a startup in the slightest, except of course that their own APIs are copyrightable and they will be prevented from parasitically cloning competitors' APIs.
P.S. There is in fact an company who has cloned Basecamp's API so they could instantly work with all the Basecamp Add-ons and more easily convert Basecamp customers to their own.
It was possible at one point to patent RESTful APIs, but no longer.
It's not clear to me how declaring APIs copyrightable impacts a startup in the slightest, except of course that their own APIs are copyrightable and they will be prevented from parasitically cloning competitors' APIs.
P.S. There is in fact an company who has cloned Basecamp's API so they could instantly work with all the Basecamp Add-ons and more easily convert Basecamp customers to their own.