I think the comparison’s a little weird because we’re talking about an impaired person behind the wheel vs. an impaired person not behind the wheel. If the argument is simply that hard drugs can make people violent (I think Walter Block refers to this as the werewolf complex), then it’s kind of puzzling why you wouldn’t make the same statement about alcohol in general. If anything, comparing how we treat hard drugs vs. how we treat alcohol would seem to suggest that hard drugs SHOULD be legal, and only driving under the influence of hard drugs should be illegal.
On the other hand, our current justice system is so barbaric and heavy handed, that I flinch at allowing the state any room to use it. Imagine what any jail time at all does to a person’s life and career and future prospects. It’s offensive to my sense of justice to allow the state to criminalize anything but the most severe crimes, as long as it treats the offenders the way it does. Sometimes, I find myself sitting in traffic, and for whatever reason, space out, or am looking in some other direction, and then I step on the gas as the light turns green only to notice a pedestrian crossing in front of me at the last minute, and I put on the brake. Then I sit there thinking to myself how my life would basically have been over if I hadn’t caught that pedestrian in my sight at that moment, and how easy it really is to become a victim of the state’s fullest ire.