If the details of moderation rules were public, companies would be facing additional labor costs dealing with posters pushing hard on the edges of those rules.

At some level, users can choose from a variety of forums with different moderation methods, or none, and companies can try to fit their moderation policies and practices to their users' expectations.

All of that develops in a cross between evolution and arms race, with lots of moving targets - global variation, changing social mores, changing technology, the latest taboos and moral panics.

Exposing the details of how the sausage is made would help the people who want to put pebbles (or worse) in the sausage more than it would help the makers or consumers. If one finds the sausage too bland, there is likely a spicier one just a few clicks away. Or one could grind one's own sausage and compete!