That's hardly always the case. For illicit drugs, they can be (and frequently are) cut with other substances in order to make more money. Sometimes those substances are benign and sometimes they spectacularly lethal. The user has no real way of knowing, and the dealer may not know either. That's one of the problems with illicit drugs, unfortunately.
With prescription pharmaceuticals, there are such things as accidental overdoses, as well. I once had a time-release pain medication that hit me all at once, most likely because the sealant on the tablet was cracked in a way that wasn't obvious. Instead of getting the medication over 8 hours, I got it in one shot. I was very fortunate that it didn't kill me, particularly since I lived alone at the time. As it was, I more or less grayed out for a few hours and my memory of that night has big gaps in it.
I definitely wouldn't characterize an overdose as a "personal choice" unless the person in question is suicidal or they're taking overlarge doses in order to get high.
Insofar as the "War on Drugs" is concerned, it's been a spectacular success for the private prison industry, among others. For the general public, not so much. For-profit prisons need to go, and America should consider what we're doing wrong, given the recidivism rates of some European countries. Our justice system has always been focused more on punishment than redemption/rehabilitation, though.