Enter Brandon Coats. Brandon has a state license that allows him to use marijuana for medicinal purposes at home. He claims his employer, Dish Network, fired him for his medical marijuana use. He therefore filed a lawsuit claiming that his employer violated the no-firing-for-lawful-conduct statute. So, does he have a claim?
Yesterday, the Colorado Supreme Court issued its opinion in Coats v. Dish Network (opinion here). The Court affirmed the lower courts' decisions dismissing the employee's lawsuit for failure to state a claim. In short, the federal Controlled Substances Act is still on the books - and it still makes marijuana use illegal (even if you have a state license). So, Mr. Coats's marijuana use was actually not a "lawful" activity because it is illegal under federal law. Case dismissed.
I should note that most states don't even have statutory protection for off-duty lawful conduct - so presumably terminating employees for marijuana use would be even easier in other states. This issue will become much more complicated if/when the federal government decriminalizes/legalizes marijuana use.