Is it not clear that drug legalization would lead to increased usage, to more addiction, to more driving under the influence? To deny this is to be a knee-jerk oppositionalist.
I'm not convinced that the issue is as clear-cut as he makes out. His argument consists of identifying four groups of people that would be more likely to use drugs if they were legalised: (A) those who respect the law, (B) those who fear legal punishment, (C) those who fear associating with criminals, and (D) those with health concerns "about the quality and strength of illegal drugs, the dangers of overdosing, etc."
This establishes that legalising drugs would cause some people to try them, who otherwise would not have done so. But that is not sufficient to establish BV's point. He must also show that this group is larger than its opposite: those who would not take drugs if legalised, who otherwise would have.
Is it plausible that some people would take drugs (if illegal), but not if they were legalised? I think so. People (especially teenagers) can be motivated by rebelliousness, or the temptation of 'forbidden fruit'. I suspect this group is indeed outnumbered by those BV describes, but I'm not sure. It's an empirical question, after all, and not immediately "clear" from the vantagepoint of our armchairs. BV's failure to even consider the alternative possibility is a glaring omission on his part - one which (to borrow a phrase) "undercuts his own credibility".
Another mistake BV makes is that increased usage does not necessarily imply "more addiction" or "more driving under the influence". It could be that removing the criminal taint from drug use would result in more responsible consumption. Greater regulation (including, say, compulsory warnings of the sort found on cigarette boxes) might well lead to more informed and cautious use of these substances. Or it might not, I don't know, but at the very least these are matters that warrant some consideration. That would surely be a better idea than making dubious assumptions and then calling your opponents names.