I agree - although I think it shouldn't be a "positive, deny, negative, accept". It should be used as evidence to either support them getting welfare (if they're clean and obviously clean), or deny.
But if they test positive, but provide proof of trying to get help (addiction isn't a choice, and quitting hard drugs cold turkey just does not work) - such as a doctor's note, a treatment program, etc., then I think it shouldn't deny them - just monitor more closely.