Could this effect actually be more of a customer service effect?

Drug dealers were lacing things with fentanyl to make them more addictive. They were putting too much in and killing people by accident. This was bad for business in 3 ways

1) they could have saved money by using less fentanyl

2) they were killing their customers, as well as reducing the customer base this has a reputational risk.

3) They were attracting too much public interest in their activities

Therefore they found that they make more money by putting less in.

Not a drug expert, don't live in US, never took fentanyl. I just picked these 'facts' out of the comments. Before anyone says, 'you don't know what to you are talking about' in the sweet way that has crept into hn, I really don't, and don't claim to.