Are the Broncos and Seahawks allowed to toke? Can an employer legally test for and tell you that you can't smoke cigarettes?
The NFL already issued a statement that marijuana was still banned, even in states where it was legal. That's not unprecedented- there are several supplements and ingredients that are perfectly legal to buy over-the-counter but which will trigger a suspension from the NFL if you test positive.
Granted, those supplements are banned under the reasoning that they're "performance enhancing", which marijuana clearly is not. Really, though, the entire "performance enhancing" boogeyman is stupid and needs to be discarded. Contact lenses are performance enhancing; they improve your eyesight from your natural god-given levels. The NFL doesn't have problem with players who wear contacts. Nor should they. If someone takes Ginko to help them remember the playbook, that's performance enhancing. All of those painkilling shots every player takes before games are performance enhancers. Coffee is a performance enhancing drug if it helps you be present and alert during morning meetings. I fail to see how enhancing someone's performance is a bad thing- I take steps to enhance my performance, and I'm sure my various employers have appreciated those steps. The NFL shouldn't be banning things under the reasoning that they make people better at their jobs. They should be banning things that are illegal, dangerous, or both. Steroids aren't bad because they enhance your performance, they're bad because if you overdo it they will cause long-term health problems and even death. It's important for the league to step in and regulate them so that the game doesn't devolve into an arms race where everyone is taking more and more steroids and the choice becomes either sacrificing your future health (because you're doing unhealthy levels of steroids) or your current career (because you can't compete against the people who choose to do unhealthy levels of steroids).