JSON-LD is just another flavor of JSON, and while I do concede it has larger applications in the open data space, at the end of the day, it is still JSON.
There are six JSON-LD tags in use at the moment, so pointing it to the JSON tag shouldn't cause very much confusion whatsoever.
Note: I came across this while editing the JSON tag to define JSON-LD as well; whatever the community decides to do, I'll edit the tags accordingly.