But if they do, a malicious actor can use that information to circumvent their restrictions, and its their walled garden, so they have very little incentive to tell everyone exactly what they don't like.
Imagine if this were applied to actual laws enforced by the police. "You're under arrest but we won't tell you what law you've broken, because then other criminals might use that knowledge of the law to avoid being arrested. And by the way, a secret court has sentenced you to life imprisonment and all of your appeals have been denied."
I'm not endorsing these US policies, but it's worth noting that even in democratic law enforcement it's accepted that the system isn't always bound by transparent policy and process. There are usually justifications for keeping some things secret and discretionary to enhance law enforcement effectiveness.
Suppose it is true that these companies can't reveal their decision making because there's so much to be gained by bad actors that game these highly centralized systems.
Then it seems like a larger number of smaller firms could be more transparent and still achieve the same effective level of security.
This is the epitome of security by obscurity.