Please see my comment to the question, which is also not a JSON comment. :-)
No, "you can comment" is not true: you can easily find it out:
http://json.org,
http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf.
When people comment JSON, they either use some actual data for commenting, or they actually use regular JavaScript comments (or any other form of comments) and call it JSON, but this is not real JSON. Such comments could be used on temporary basis, filtering them out before feeding "real" JSON to algorithm compliant with this standard. In other word, using some tricks which are never real JSON comments, which don't exist.
Why so? Please see my comment to the question again. :-)
—SA