I guess by low level he is not talking about the marijuana stores and dealers in 20 States that his department refuse to prosecute for breaking the Federal law.
In addition to being a political hack, Holder is an idiot.
The threat of prison as a penalty for criminal prosecution has forcefully persuaded many people to get addiction treatment. Some who would never take it of their own initiative take it in a prison-or-treatment context, and go on to lead clean and sober lives. Even those who do not take it to heart sometimes live clean while they are under court supervision, and some of them learn principles of sober living to which they will return later, when they have hit bottom and sincerely want a clean and sober life for themselves. Any step toward sobriety, any time of drug-free living, is to the good.
But, if there is no prison threat, all of these prison-or-treatment choices are thrown out, and people who were not ready to seek treatment of their own accord will go through more personal hell and hurt to those who love them before they will do what is needed to get better.
Many of my clean addict friends will say, "I just started learning what an addict needs to do to get better because the court required it, and it was easier than going to prison. But then, it took hold in my life, and I found out it worked. Now I've got a new life."