From the FAQ:
JSON API has no requirements about URI structure, implementations are free to use whatever form they wish.
This seems to me a critical feature of JSON API because it decouples the server from the client. The client has to rely on explicit related links given by the server (which can change at any time), not on stitching the URLs together according to some predefined templates. If the server provides no related links, this means that the server doesn’t support browsing that relationship.