The Corleones leaving drugs doesn't make any sense. Vito made the peace based on his protecting drugs in NY. There's nothing further in I, II, or III about the Corleones abandoning that protection or drugs generally. Zasa's statement about "I'll kill anyone who does" only applies to "the neighborood" as one grandmother complained, not the greater NY area. I don't know. Perhaps we are meant to understand that Michael, in his eternal quest for "legitimacy", used his influence to eliminate drugs by the Corleones.

Sloppy writing? For sure.