Return value of unitag_taxonomy_terms_resolve_tags() is inconsistent

I am integrating unitag with community tags and am calling unitag_taxonomy_terms_resolve_tags() in order to do so. If the vocabulary is configured as read-only the 'parsed' terms are returned indexed by their tids which is what I would expect. However, if the vocabulary is not configured to be read-only then the following code means that the parsed terms are merged with new terms and tid indexes are lost:

  if (!in_array($vid, $unitag_config['readonly'])) {
    // Add new terms - use array_merge to ensure that numeric indices are
    // not overwritten.
    $tags_parsed = array_merge($tags_parsed, $terms_new);
    $terms_new = array();
  }

Is this intended or a bug? The comment in the code suggests it is not intended but then I can't see why new terms are merged with parsed terms instead of being returned in the 'new' array. As it stands, I have to iterate through the parsed terms and repeat the existing term test.

I should point out that it works fine with the node edit form. It's just that as an API call (which you may not have intended) it doesn't do what I would expect it to.

Andy