I think it's pretty obvious there was no narcotics ban. The Commission met when the new drug laws went into effect and decided to outsource the trade to the Sicilians except that their personal investment was involved. You might have a few capos in a family that report directly to the boss and move the junk (Joe the Blonde?). Everyone else was instructed to stay away due to the draconian laws passed against trafficking. When it was time for a new boss, the guy selected was likely the "drug capo" all along. Carlo Gambino was always connected to the Sicilian mafia long before becoming boss.