Why?

Intent is less clear, and even if prisons in the US were much more humane, addicted people would need extra resources (including human effort) to break the addiction and hopefully prevent further incidents. Really, addicted people are a liability because they'll do pretty much anything to get more of their drugs (and they're often violent); it's just a matter of how prevalent this has to be to be a serious issue. Of course, they're not solely responsible for pervasive drug usage.