Doesn't this kind of defeat your whole point? People with a drug problem will have a drug problem regardless of the drug.

People are impaired when they take mind altering substances. An employer can fire you for coming into work drunk, and I don't see why coming in to work high should be any different. That being said, I do think it's a bit silly for an employer to weight being a weed user any differently than being a drinker. But... it's not like employers don't take drinking habits into account when making hiring decisions. If you tell your employer about how you like to party on the weekend and get smashed all the time, they are well within their right to not offer you the job. I'm not sure why they should be required to treat alternate drug use differently.