I can see why you'd ask that question, Goodfella. I think the answer is that it's a Sicilian term denoting that he's the head Mafioso in the district--the equivalent of one of the heads of the Five Families in New York, hence, "Don." Puzo several times subsequently refers to Don Tomassino as "the Mafia chief" or "chieftan," which settles it for me. However, in Italy and Spain, "Don" is an honorific that can apply to any "man of respect," even if he's not a Mafia chieftan.