While I agree with you that in general there's nothing wrong with RPC, I'm not sure what the advantage in this case is. You seem to have an idea, though…?
Oh, you'd have to ask the JMAP devs. But I do think they'd have to bend the constraints pretty hard to have a RESTful interface while keeping the same advantages w.r.t. efficiency in the number of requests and data transferred.
Like Fielding writes in his dissertation, "The REST interface is designed to be efficient for large-grain hypermedia data transfer, optimizing for the common case of the Web, but resulting in an interface that is not optimal for other forms of architectural interaction."
