I've seen that, and even been "guilty" of it myself years ago - many people take 'RESTful' to just mean anything that returns JSON instead of a full HTML document.
[1] https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arc...
That said, JSON is just an encoding format. The actual response format is not "JSON", but the specific structure that your clients are expecting to receive (even if it is encoded in JSON), and that can certainly have links.