Re: Depends on your clearance level
Because drugs are only used by commie hippies, so if you use drugs you are a commie hippie and probably a member of ISIS.
The 'official' reason is that you might be blackmailed to keep the information that you had used legal weed secret, but that is only an issue if you get fired for using legal weed.
It's like not hiring gays because they are a blackmail risk if anyone threatened to reveal that they were gay.
Biting the hand that feeds IT
