If a product is more widely available & readily sold, then there are going to be more people addicted.
Criminalization has nothing to do with the process of somebody becoming addicted to something. Heroin(& friends) are addictive to people who try them because they give immense pleasure. When the pleasure becomes less, people don't stop the drug, they take larger doses of it. And then if they're stuck for cash, go for something cheaper that still has pleasure. Isolation may play a part but it is a small part.
from https://www.health.harvard.edu/newsletter_article/how-addict...:
"Addictive drugs provide a shortcut to the brain's reward system by flooding the nucleus accumbens with dopamine. The hippocampus lays down memories of this rapid sense of satisfaction, and the amygdala creates a conditioned response to certain stimuli."
> Something being legal takes a lot of the desire to do something away
That is just not true. What is that statement based on? People want to experiment with things(either purely for a new experience or to conform with their peers).
If product x gives you a high, you will want to try it again..and again..and again. This isn't theoretical, we see this issue with heroin, opioids, crack cocaine.