Yes, it is about blackmail. At the start of the 80s, smoking weed or being gay was OK with DISCO (Defense Industrial Security Clearance Organization) as long as you came clean about it and (for example) did not try to hide your sexual orientation from your friends & family. Because therein would lie the opportunity for blackmail.

This attitude of tolerance eroded later in the 80s. During the Reagan regime, they (DoD?) sent out a memo saying that marijuana use was "incompatible with national security goals". Not sure what the effect was out in the trenches though.