From President Obama on down, politicians are saying that America needs to stop locking up so many people. And from President Obama on down, they're saying that the way to do that is to reconsider the way we treat nonviolent drug offenders — to stop giving them long sentences, and to offer them alternatives, like drug treatment, so they can deal with the underlying problems that drove them to crime.
It's a view that has long had public support (even at the height of the war on drugs). The public believes that "nonviolent drug offenders" are just a different kind of person than real criminals, and politicians are playing into that to encourage criminal justice reforms that target nonviolent offenders and rely heavily on a treatment-heavy approach to drug prisoners.
If you believe all that, it's easy to believe that the prison problem is easy to fix. But it isn't — because there aren't that many nonviolent drug offenders in the grand scheme of the criminal justice system, and because there isn't as firm a distinction between "nonviolent drug offenders" and everyone else as the public likes to believe.