Any substance abuse website that does this is run by charlatans and has all the morality of a hyena. This is such a violation of trust that I cannot imagine any healing coming from it.
Or is there something else that makes this bad that I'm missing?
"This seems generally positive apart from the risk of occasionally ruining someone's life."
Your profile says you work for Google. Maybe Google has an "enlightened" policy towards substance abuse, but I wonder what your manager would do if s/he thought you were in need of rehab? In a lot of companies, getting rid of you would be the manager's reaction. Starting the death spiral for someone who is looking for help is horrible and to treat it like any other targeted advertising is insensitive at best.
Given the down votes, I would guess people agree with you, but I really wish our profession would think about these things and the affects on normal people.