Imagine you were pursued for months by the government for crimes you didn't commit - or for actions that, at worst, most people wouldn't know were criminal, and that most people wouldn't even know were criminal. Now imagine that government officials overcharged you, so that in the face of hundreds of years in prison, the option of pleading guilty to the lesser crimes and a comparatively light sentence was an offer no reasonable person could resist.
So far, this is unfortunately common in the criminal-justice system. But now imagine one more twist: In exchange for allowing you to plead to the lesser crimes that bring the lighter sentence, the government also swears you to secrecy.