It would have been a bad deal for the Corleones. Forgetting Vito's "personal" opposition to drugs -- he was personally against prostitution also --- he knew it was not a good business for the Corleone family. The reason he gave, that the political, police and judicial protection would dry up was only a part of it. The structure of the Corleone family was not geared to drug dealing. They ran unions and protection rackets, and probably gambling, and some prostitution. What they lacked was a "top narcotics man" like Solozzo who was already on another payroll. If Clemenza and Tessio had to be in the drug business, their territories would have been chiseled away by the other families, just as they were adfter Vito said "yes" to drugs.