I'm personally not a fan of marijuana, but can certainly understand why some would prefer using it for medicinal purposes over prescription pain medication that can become highly addictive in very short order.
The legalities of state legalization vs company policies with work, however, seem a bit complicated in nature to me.
Beyond that, it's none of my business if people partake or not. Just because I don't care for it doesn't mean I think it's worse than a lot of other options people have available.
But again, I do realize that it's not unreasonable for employers to uphold certain standards regarding employment, despite states that have/are considering legalizing it. Perhaps there's technology available (or will be) that can differentiate between recent use and current use on an hourly basis? Otherwise I personally have nary a clue how that all gets resolved.