Yes, it's in the supply because it is in high demand. It also allows the dealer to offer lower prices to which the market is very responsive.
Regarding alcohol, people die every day from alcohol, and many are poisoned to death, or blinded, or permanently disabled due to alcohol abuse or ingestion of alcohol not meant for human consumption.
All of that's true. But this conversation started with a 12-year-old dying from an overdose from tainted supply (at least the way I read the story). I don't know anything about the kid or her mother, but I'll stipulate that both of them made poor decisions for the sake of this discussion.
But if that 12-year-old had snuck out to drink alcohol, or smoke cigarettes, either of which would also constitute an illegal activity, and had then died because she drank poisoned beer or tainted cigarettes, we would definitely be interested in how the products had been poisoned as a key element of our discussion.
Drug addicts should be guided into rehab. I'm not sure that some level of stigmatization isn't appropriate to encourage that as it seems to have worked with cigarettes (and nicotine is very addictive). But people have to be alive before they can be rehabilitated, and I think exploring ways to eliminate unsafe supply is a relevant part of that conversation.