1. We'd treat drug using employees really seriously
2. Therefore you must not do drugs
3. Because we're so serious about this this becomes a major blackmailing vector.
4. So we need to make really sure you're a drug user.
It's an entirely manufactured problem that could be solved overnight by them just saying "hey, don't show up high on the job, but what you do in your own time is your own business, and we won't pro-actively investigate it anymore than any other citizen".
Don't seem to recall being blackmailed for drugs coming into it
Applying to 18F didn't require a drug test for me. The application process asks this question in the background check part (details at "security clearance" at the bottom of https://pages.18f.gov/joining-18f/how-to-apply/ for anyone curious), but answering yes to the "have you ever done federally-illegal drugs" question is not an automatic disqualification. It can be a disqualification depending on the details of the drug use.
Why is that?
I mean, this would be a case of "shutting the stable door after the horse has bolted" except that Snowden would pretty clearly have passed a more thorough background check. His background wasn't the problem.
The only thing I can think of that might actually have preemptively filtered him (or those like him) out would be some sort of psych test biased toward authoritarianism.
Background checks are obviously a necessary and useful tool, I am just not sure I understand which dotted lines (regardless of their logic or illogic) connect "Snowden" to "more background checks" in a knee-jerk reaction.