Well, for one thing, it policy of the courts for use the prison/treatment option when one is found in possession of as small a quanty as 3 grams, and suprise!, the great majority of people opt for "treatment". This is a matter of public record. How is it you don't know; don't you ever venture out to actually do research on your position in an issue?
So, it's your contention that the vast majority of the people involved in this study are people who have been ordered by the court to receive treatment.
Boy, the extent that pro-potties will go to in order to trash anyone who disagrees with you. Awful Democratic of you.