My correction regarding the study being published in 2014 was removed.   I used a term used elsewhere in the comments section, but I chose a different ethnic group to apply it to.   My bad.

I suspect all people use drugs and alcohol for the same reason - to self medicate, to recreate, to alter consciousness.  It would be pretty racist to attribute alcohol and drug abuse to being employed.  It would be one more attempt at making the behavior a virtue if I do it (because I work), and insisting that others who do the exact same thing are inferior.   This position completely ignores the damage that drug and alcohol "abuse" (to quote you) has on the people in the abuser's life, and elsewhere in society, including driving fatalities and injuries and domestic violence, whether the abuser "works" or not.