All users are allowed to offer MM services. The OMM rank is simply a symbol of the staff's trust. If, for whatever reason, an OMM loses said trust, the rank is removed. It would be counter-productive to give them an ex-OMM rank instead, since it would offer the same implications of trust, especially if shared by voluntarily retired OMM's.
Moreover, OMM's get paid for their services; it is not generally regarded as a generous contribution to the site. Hence, when mods retire (or get demoted), they sometimes get a rank to recognize the work they've done, but for OMM's, their fees are their compensation. Since most of them have dedicated MSN addresses for MM'ing, they usually only have to stop using their public MSN if they don't wish to be bothered, or else put a simple note in their MSN status or Sythe profile indicating they are no longer offering their services.
