He argues (convincingly) that all drugs should be decriminalized. Putting someone in jail is the most extreme thing that our society does, and needs a serious justification. Every individual being put in jail deserves an answer, and no satisfactory answer can be given for putting someone in jail for a nonviolent drug use (or possession).
TL;DR version: anyone being put in jail deserves an explanation. None can be given when they are a non-violent drug user.
Surprisingly, the prohibition of alcohol in 1920's did not punish alcohol drinkers, just distributors. This model for drugs today would be a major improvement over the draconian treatment we have of the non-violent drug offenders.
The gist: whenever someone is about to be put in jail for any reason, they ought to be given a reasonable answer to the question "Why am I being put in jail?" There simply is no satisfactory answer for current drug charges.
Bad answers to this question would require, out of consistency, criminalization of alcohol, tobacco, skiing (dangerous!), pizza (bad for your health!), etc.
I highly recommend the book Legalize This!: The Case for Decriminalizing Drugs (Practical Ethics Series) by Doug Husak