Because it's the right thing to do and I doubt it really has much influence on liability unless you say something like "we're not working with you because you're a black woman".

I watched a documentary called “Suits” a while back about a corporate legal firm and that convinced me that you are wrong. Those well dressed professionals will use anything you say against you in the court of law!

https://www.imdb.com/title/tt1632701/ is a documentary?

I am afraid you failed to read the fine print ;) I think the average legalese boilerplate is along the following lines: All characters and events depicted in this film are entirely fictitious. Any similarity to actual events or persons, living or dead, is purely coincidental.

You're replying to obvious sarcasm.

Poe's Law

Because of the scale at which Facebook operates, and the various antitrust etc investigations constantly surrounding it, they probably have every reason to be circumspect in such cases. As much as I am personally in favor of transparency, I can understand why Facebook thinks it's not in their best interest to communicate any reasons for severing business relationships. Anything they say can and will be used against them, and pretty much nothing they say will benefit them.