By special request.
Isn't the same true for alcohol? As far as I'm aware, there's nothing that prevents an employer from prohibiting alcohol use on your own time and firing you if they find out that you're doing it. It would just be a dumb thing to do. Hopefully they'll figure that out about marijuana sooner or later.
The marijuana issue is fraught for another reason: state vs federal laws and regulations. It would be surprising to me if any company with federal contracts could look the other way on marijuana use under the Drug Free Workplace policies.