It's proven to work in terms of liability in majority of cases.
So, I believe that the FBI and NSA might have worked together to use legal threats related to Patriot Act to force companies to comply with SIGINT-enabling for collection purposes. The other ECI leaks mention U.S. companies that were cooperative and made their systems "exploitable" for "SIGINT-enabling." So, they offer money first and call the FBI if that doesn't work. Almost everyone caved so I'm guessing there's a significant, legal threat there.
They're not telling you in detail because it's classified: releasing the info is a felony. Who would after seeing what happened to other whistleblowers... It's why I recommend privacy-focused companies being located in Iceland or countries similarly non-cooperative with police state activities. At least one can legally resist subversion in those countries rather than experience... whatever FBI does... for not subverting one's products.
[1] https://www.techdirt.com/articles/20130614/10341723470/yahoo...
[2] https://firstlook.org/theintercept/document/2014/10/10/eci-w...
[3] https://www.nsa.gov/public_info/_files/speeches_testimonies/...