Sure. And? I am a federal contractor. But my employer can still set standards if it wants, including for things like security clearances.
Alcohol is legal, and it is perfectly legal to be a drunk. But good luck getting a security clearance if they figure out you are an alcoholic. I am not allowed to be drunk at work, even though drinking is legal. I get called in for random tests that include alcohol. If marijuana is still on their list, then I still risk my career by partaking, legal or not.
Bottom line, if the feds decriminalize or legalize marijuana at a federal level, I highly doubt anything would change for employers like mine. But it would make it easier for people in the business to do banking and pay their taxes.