I see some problems here.
1. Drug users often do commit crimes to support their habits, but this is largely due to the fact that the black market status drives the price up to a ridiculous point.
2. From the article: "Imprisoning large numbers of people has some effect on crime, but there is a point of diminishing returns," argued the Sentencing Project's Mauer. "Initial research shows that maybe a quarter of the decline in violent crime is due to incarceration, but that means three-quarters isn't. The rest has something to do with a relatively healthy economy in the 1990s, the reduction in crime and violence associated with the maturing of the crack cocaine epidemic in the early 1990s, and the efforts by police in some cities to reduce the flow of guns."
3. I don't know whether or not there are any statistics to back this up, but some have argued that due to the large prison population, violent offenders must be released in order to make room for nonviolent drug offenders.
Another issue here is the degree to which drug prohibition actually increases crime. Work has been done showing a higher murder rate due to prohibition.