The point is that (as I understand it, which may not be that well) folks on that side tend to frame this fundamentally differently than you do. They don't view a system of laws as an evolving structure to balance rights and harms, they view the legal code as a formal statement of society's shared moral standards.

So if drug use is considered immoral (which, keep in mind, has been the standard mainstream message since before almost all of us were born), that philosophy would imply that society has a moral duty to declare that publicly and to punish those who practice or promote immorality. (And naturally, to address your last point, violence done by the societally approved defenders of morality (i.e. the police) is more acceptable than violence done by people actively breaking the rules and thus the common moral code.)

I won't swear that I have any of that exactly right, but it's the sense that I've gotten from many conversations and articles about this sort of thing. I pretty strongly disagree with their premise about the purpose of laws, but within their framework I think their behavior is reasonably sensible.