It's illegal substances for one reason only. When someone is taken to court by the government (in the US, that is), the man or woman hires or is appointed an attorney. This makes him an infant, a ward of the court, like an insane person, or one who can't handle things on his own. In other words, he has given himself over to whatever the court decides, and essentially can't even speak except that the court lets him.

See https://www.youarelaw.org/Download/CorpusJurisSecundum-AttorneyClient.pdf.