JSON isn't semantic so there is no need to put __ before comment, it's not like it's meta or something. It will still be parsed and processed by the JSON parser, which is a waste of computer cycle.
It's kind of a future-proofing thing. If you put a field called "comment" in a JSON blob, especially one in a format you don't control, you run the risk that future versions of the format will define the "comment" field and give it actual meaning. A crazy prefix makes this at least slightly less likely.
