Makes it seem more like damage control than vetting candidates.
Wouldn't the state department want to know if an individual has issues dealing with confidential information?
If something like that comes up in a background check - i.e, posted links on facebook - it would be a good clue that the candidate should not be hired, at least according to these terms. (Never mind whether commenting on a big news story really means anything about ability to keep stuff secret)
But if candidates are specifically told "Don't do this, or you won't get a job", then their compliance with the suggestion no longer reflects their attitude towards confidential information in any way, just their level of stupidity. Or defiance, I guess. But not how good they will keep secrets.
In any case, it's being presented almost as a vetting procedure to filter out unwanted candidates, but the goal seems to really be trying to stop people from talking about it in general. And who can blame them. Although I think that's a losing battle.