There is a difference between disagreeing with someone's estimation of costs versus benefits, and finding their reasoning "perplexing." In fact I think the benefits of criminal drug enforcement does not outweigh the cost of enforcement. But I can easily see how someone might come to the opposite conclusion. Crack babies aren't a made-up problem. Many homeless are drug users. Meth really is ripping apart families in rural America. All you have to do to conclude we should make those substances illegal is underestimate the massive cost of drug enforcement. And that's not an irrational thing to do--its not like either side has concrete numbers to point to.
He's pointing out that we have empirical evidence for the harm of certain substances and it's a different kind of harm than the largely unintended consequences of, say, cars, which provide us with far more direct advantages at reasonably low risk. The risk is also intentional and, to a degree, controllable with wild animals and intoxicated drivers being some of the major exceptions. (Inclement weather is at least partially controllable, as long as "don't drive" is a choice.)
But we also do have empirical evidence for the harm from cars. Thats what your parent is saying. The scale of harm is very minuscle when compared to car accidents. Even wars are less deadly than cars when just looking at the numbers. The reason you think intoxicated drivers are exceptions or that accidents are unintended consequences is that, in peoples minds accidents cause less emotions than the image of a poor baby with cockroaches with parents on drugs.
