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.
The author agrees with you that further study is merited.