The rule, in reality, is don't get caught dealing drugs.
All of the Bosses know the sources of the fat envelopes,but their main concern is not appearing to sanction that sort of activity. This way,no testimony or wiretap evidence will ever surface to incriminate a Boss. Every soldier and capo on the street knows to keep their dope activity on the QT.
The severity of the penalties,as well as the public outrage guarantees that anyone who is stupid enough to even mention drugs while handing over an envelope would not live to see the next sunrise.