The notion of a "promise" when it come to a Callbacks object doesn't make a lot of sense. Callbacks are low-level objects that already provide a lot and we don't intend to complexify their API even more.

Keep in mind Callbacks methods are "detachable", exactly like Deferreds methods btw. So you could create a new object with the methods you want :

function cbReadOnly( callbacks ) {
  return {
    add: callbacks.add,
    remove: callbacks.remove
  };
}