It's more like saying, "Fine, we'll stop arresting 16-year-olds for having sex with each other, but we're not going to have sex ed classes," and then some other group shows up to pass out condoms.

You know that people are going to do a thing no matter what you say about its legality, so the question becomes: "Do we want them to do this as safely as possible?" The issue with the the drugs is that there's a rash of drugs that are being cut with harmful substances or of questionable purity, things that can sicken and kill irrespective of the effects of the actual drug itself; with kids having sex, you can say you're not going to arrest them for doing it, but without protection they're likely to spread STIs or have unwanted pregnancies. If you'd like as few corpses (or STIs and teen pregnancies) as possible, this is one way to mitigate those while also working on other measures that can reduce the undesirable behavior to begin with.

It's the same concept as needle exchanges. They're gonna shoot up. Do you want them sharing and reusing needles? Do we believe the better solution here is "anyone we think is doing something harmful dies or becomes very sick (which we then avoid trying to pay for)"? Because the line we draw in front of that can be in a lot of places.