It important to remember that Edward Snowden was a contractor. Why did he work for NSA as a contractor instead of a regular employee? Because he had no degree. Such people are generally shunned by HR managers. But if they have the right skills, hiring managers will often use contractor status to circumvent their own HR dept.
Thanks to a number of lawsuits, most employers have mandatory time limits for contractors, typically 1-3 years. Although many employers promote their best contractors to regular employment, HR often balks at waiving a degree requirement, even for people who are doing excellent work without one.
All contractors know (or should know) how much time they have on the clock. Once they understand the time limit, every contractor needs an exit strategy in case the employer declines to offer full-time employment when the time limit expires.
Whatever Snowden did, he did it with the understanding that his time at NSA was limited, and crossing over to full-time employment was going to be a challenge. In other words, he had nothing to lose.
If employers didn't have to use contract employment to circumvent their self-imposed budget and HR obstacles, you wouldn't see so many contractors with incentives to take secrets put the door.
I'm wouldn't be surprised to see this scenario repeated many times at the NSA.