You got exposed to such substances because the law enforcement failed to apply the rules against controlled substances.
Despite this, the fact that the substance is illegal or controlled, made that less people have been exposed (and eventually addicted) than if the substance was freely circulating.
Singapore was well known to have a strong opium problem. Now they have death penalty, no opium problem anymore.
Clear example:
If cannabis cakes (space cakes) were legal, I'd consume them but they are not, and I do not trust + do not want to fund dealers. So I don't buy them.
Another example:
Alcohol is forbidden to 12 year old kids. Yes, some of them may find a workaround, and a way to buy it, but because of that you are still helping a large segment of the kids to not get early into addiction.
If tomorrow you say to the kids that they can drink wine (in French schools it was possible before for kids!), then they are more likely to get addicted.
Yes, there is still a small % that will get exposed, no solution covers 100% of the population, but if you can save 6 out of 10 addictions by regulating the substance, then you are doing a good job.