Well, it isn't a very good rehab center :D All those people coming and going, sneaking new stuff in... And Elrond needs to relax every so often, so he might overlook, say, Gandalf bringing some pipeweed in as long as he gets some.

ETA: The other problem is Elrond's major conflict of interests - Gil-Galad left him in charge of one of the three Elven Drug Rings. Preserving all the pretty Elvish strongholds is expensive, after all. I think we decided that's why Elrond opened the rehab clinic in the first place: he felt bad about profiting off turning his people and others into tra-la-la addicts, so he figured he should at least keep them from getting too bad off.