Fed's med-marijuana ruling: Does it affect workplace drug bans?

The U.S. Justice Department has announced it won’t enforce federal laws banning marijuana use for medical purposes, and some states have OK’d prescription pot. So, how do those rulings affect workplace anti-drug policies?
Most importantly, if your firm bans the use of marijuana — medical or otherwise — as a straight safety precaution, such as for vehicle drivers or employees who work around dangerous equipment, you’re probably OK with the ban.
There’s no definitive court ruling on that, but there is a federal directive regarding a Department of Transportation ban on medical marijuana for drivers. In summary the DOT, responding to inquiries resulting from the DOJ ruling, has announced, “DOJ guidelines will have no bearing on the Department of Transportation’s regulated drug testing program. We will not change our regulated drug testing program based upon these guidelines to Federal prosecutors.”
Translation: If it’s a safety issue, we’re still enforcing the regs against marijuana use. (Under DOT’s regs, the ban extends to a wide range of transportation employees, such as pilots, school bus drivers, truck drivers, train engineers, subway operators, aircraft maintenance personnel, transit fire-armed security personnel, ship captains  and pipeline emergency response personnel.)
Employers in states that allow medical marijuana probably can take their cue from that ruling: When safety’s involved, you can test and ban.
The effect of the ruling is less clear if you routinely drug test — with no safety issue attached — in a state that allows medical marijuana.
There are 13 states that allow medical-marijuana use: Alaska, California, Colorado, Hawaii, Maine, Michigan, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont and Washington.
Two states — Arizona and Maryland — don’t fully allow medical marijuana but have laws favorable to its use, such as allowing it as a defense in court.