No one is arguing that Alice and Bob should be prevented from engaging in good ol' American trade.
We're arguing that when drugs are ubiquitous -- as easy to access as pushing a button -- you'll spawn far more addicts than would have otherwise been created.
When an addict's life is destroyed by these horrific tools of pleasure we've created, are they going to take solace in the fact that they were able to use technology to enable their habit?
People become addicts for many reasons. One of the main ways is by being at a party among friends. No one expects to be addicted to this stuff, so they try a bit.
In the old days, they had one option: Get more from their friend, which may not work forever. In modern times, they can get as much as they want from the darknet markets. Bitcoin ATMs make it trivial, too. You don't even need to be vetted by Coinbase.
It's not just theoretical. People have witnessed the markets causing this.