@manton it’s not that they agree to tell the truth.
It’s that they agree to tell prosecutors what they want to hear.
Such exercises of power and plea bargaining have always been corrupting.
Just recognize: you’re beginning the question when you say they tell the truth. If we knew already that what they’re saying was the truth then it wouldn’t matter if they said it or not.
What they’re really getting here are confederates operating under threat of harsh punishment of the state.