Depressed people and diabetics are hurting themselves either through action or inaction, but I've never been threatened by a diabetic or depressive outside a Circle K. I have been threatened by a criddler outside a Circle K. Similarly, few people get all of their stuff stolen out of their house by a diabetic or depressive. They do all the time when their son is addicted to heroin. Depressives don't prostitute their kids to drug dealers for crack, etc.
Drug addiction is one of the few diseases where the patient actively attacks all of the people who are in a position to help him. That generally alienates people who tend to be otherwise generous toward the down-and-out. The laws reflect that, and it takes a superhuman effort of rationality to say, "Yes, addicts do terrible things. Other solutions produce better results than the natural impulse of doing terrible things back to them."
Edit: We see this in the nastier forms of mental illness, too. People are generally compassionate toward those with OCD, eating disorders, and so on. Those with borderline personality disorder or bipolar disorder tend to be relegated to "batshit insane" and viewed with utter contempt. Again, the difference is that someone with anorexia nervosa isn't going to pull a Carrie Underwood[1] on your car, but someone with BPD might.