[nradov]

In the Venn Diagram of "People who use or sell addictive drugs" and "People who commit other crimes" there is a very large overlap between those two sets. Law enforcement doesn't always have resources to investigate those other crimes but when they catch someone in possession of illegal drugs it's an open-and-shut case. So that serves as a useful proxy to keep habitual criminals locked up and prevent them from committing additional more serious crimes.

Personally I think that logic is twisted, and we should decriminalize drugs to focus on addiction treatment instead. But there's your explanation.

[RangerScience]

Did you intend to change language from "addictive drugs" to "illegal drugs"? Many non-addictive drugs are also illegal.