I'm hope I will be corrected if I am in error, but off the top of my head, I think I remember either reading (book) or hearing (movie) something to the effect that drug convictions carried much longer prison sentences than gambling or prostitution. Therefore, the Corleone's influence with judges could result in light sentences if a drug runner was convicted. A lighter sentence meant that these drug runners were less likely to cop a plea bargin for nformation that would lead to higher up members of the families being betrayed, arrested, and convicted to hard time. (or a reasonable facsimile thereof)