Non-Disclosure "Agreements?"

Why are CIA people being asked (ordered) to sign non-disclosure agreements about the Islamic terrorist attack on the embassy property in Benghazi? Can those agreements be enforced? If so, maybe robbers and murderers can get their victims and witnesses to sign similar agreements, and enforce them. How is it that guilty politicians can force people to sign these promises not to tell what they know about that politician’s crimes? Boy it sure would be nice for CRIMINALS to be able to force their accusers to do likewise as it is for crooked politicias. It sure would make for a lot fewer convictions for the crimes they commit. Why do we allow politicians to do this? Because they have made laws to allow it, knowing they’ll need them later? (Fox News)