taxonomy_vocabulary_save creates fields for new vocabs but Field UI never displays existing fields which don't have instances

In writing help text for taxonomy module, we found that newly created vocabularies trigger the creation of taxonomy term fields named taxonomy_MACHINENAME. However, without instances, these fields are not listed as "existing" in the Fields UI. This means users must create fields that are redundant with those already created but which are hidden.

Either this is a taxonomy module problem or it's a field UI problem.

See field_ui_existing_field_options().
See taxonomy_vocabulary_save().