$.getJSON() isn't automatically appending "&callback=?" to my cross-domain URL

$.getJSON() isn't automatically appending "&callback=?" to my cross-domain URL

The $.getJSON() documentation states:
If the specified URL is on a remote server, the request is treated as JSONP instead. See the discussion of the jsonp data type in $.ajax() for more details.
The $.ajax() documentation for the jsonp data type states (emphasis mine):
Loads in a JSON block using JSONP. Will add an extra "?callback=?" to the end of your URL to specify the callback.
So it seems that if I call $.getJSON() with a cross-domain URL, the extra "callback=?" parameter should automatically get added. (Other parts of the documentation support this interpretation.)

However, I'm not seeing that behavior. If I don't add the "callback=?" explicitly, jQuery incorrectly makes an XMLHttpRequest (which returns null data since I can't read the response cross-domain). If I do add it explicitly, jQuery correctly makes a <script> request.

Here's an example:

  1. var URL = "http://www.geonames.org/postalCodeLookupJSON" +
  2.     "?postalcode=10504&country=US";

  4. function alertResponse(data, status) {
  5.   alert("data: " + data + ", status: " + status);
  6. }

  8. $.getJSON(URL, alertResponse);
  9. // alerts "data: null, status: success"

  11. $.getJSON(URL + "&callback=?", alertResponse);
  12. // alerts "data: [object Object], status: undefined"

So what's going on? Am I misunderstanding the documentation or forgetting something?

It goes without saying that this isn't a huge deal, but I'm creating a web API and I purposely set the callback parameter to "callback" in the hopes of tailoring it nicely to jQuery usage.

Thanks!


(This is a duplicate of a Stack Overflow post that hasn't gotten any replies so far, though it did get two votes!)