Well, what about the value of making clear to people that in case they fall over that edge and start using drugs, they will be punished? Police being able to arrest people and caution them, rather than being told to actively ignore it?
In order to stop them from tempted to try it or go further, and prevent more people from joining that group? Or the people who deal drugs? Which decriminalization basically removed any check function on?
The material problem is that their lives are often so miserable that feeling good from doing drugs even under threat of incarceration overrides most punishments you could dish out.
In order to stop them from tempted to try it or go further, and prevent more people from joining that group? Or the people who deal drugs? Which decriminalization basically removed any check function on?