Let me play the devils advocate here..

How are they supposed to investigate crime if nobody talks to them?

As an analogy, Punching Jim down by the corner store punches everybody in the face who talks to him, so everyone has stopped talking to him. But how is Punching Jim supposed to fulfill his job as a therapist if his patients are constantly afraid of being punched? The answer is that Punching Jim might just be a bad therapist, and maybe it's good for therapists that punch people in the face to have fewer patients.

Nobody forces the FBI to have the system they have. We could have laws that make interrogation different. We could have laws that change the consequences of accidentally lying to federal agents, that raise the legal standard for consequential lies. The FBI could get rid of the no-fly list, they could stop threatening innocent security researchers to get info. We could get rid of "gotcha" interrogation techniques that are explicitly designed to try and trick people into signing away their rights.

They built this system, and nobody but them is to blame for it. If the FBI wants people to talk to them, they just need to stop punching people in the face. A lack of informants is a problem they created and that they can solve.

That's not really devil's advocate, just a great example of the many corrosive effects of an untrustworthy government.

I speak for myself here, but it is not that I would recommend against talking in principle. Only, in practice the system is set up for talking to be universally a bad idea.

If the system puts in place some better checks and balances for people to fight abuses (as the one in the post on top), I will gladly go back and do my civic duties to stop crime.

So, how are they supposed to investigate if nobody talks? They are supposed to fight for better checks and balances, so people can start talking again.