Not only is that not completely missing from the paper, it's addressed empirically. The author is particularly concerned with the affect of prior drug arrests on felony charging, with previous drug convictions as a predicate for three-strikes laws, and with the relationship between violent offenders and drug charges --- they turn out to be mostly disjoint cohorts.
The author agrees with you that further study is merited.
That's a nice start, but many members of a drug gang might not ever touch drugs, at least if I can believe The Wire's portrayal of drug organizations and their division of labor. I am also skeptical of the correlation between arrests for drug offenses (where there is no victim to complain) and violent crimes.
The author would be the first to agree that there are questions raised by drug prohibition that we don't have enough data to answer. You should give the paper a close read. It's hard to come away from it thinking that he's blasé about prohibition.
The author agrees with you that further study is merited.