It doesn't. It's partly an acknowledgement that, "It's really none of your business what people put in their own bodies."
You're implicitly saying, "I don't like the fact that you are addicted to X substance or action Y, so I think that gives me the moral justification to lock you in a literal cage for a very long time."
Legalizing drugs is about not putting people in jail for harming their own bodies and providing them with safer alternatives (IE, actual heroin instead of fentanyl, which is why we have this so-called "epidemic")
And for the record, never tried heroin, never will, it ruins lives. But I don't think we need to be putting people in jail for it. In fact, this only makes it worse.
Btw, ever wonder how much fentanyl it takes to kill you vs the amount of heroin? https://www.statnews.com/2016/09/29/fentanyl-heroin-photo-fa... That's what's driving the huge number of deaths from "heroin".