Mos pro lege (usage has the force of law).
Mores are customary usages (unwritten laws). Such was always the case with the tiara on the papal coat-of-arms. Today there is some unecessary confusion as we sometimes see the mitre put in place of the tiara [as seen here] which clearly comes from a confused idea of collegiality.
With the death of Pope John Paul and the quick election of Pope Benedict the new arms of the newly elected Pontiff was printed on a square of white with no tiara and was simply attached over the former embroidered arms of tiara, keys and crest of Pope John Paul [as depicted in photo].
It has been five years already. Commission a new drapery with the new arms of the newly elected Pope embroidered with tiara, keys and crest and tell people why (cf. Lumen Gentium Chapter III: The Hierarchial Structure of the Church).