One common situation is that player A reports player B for something, GM punishes B, but telling B what caused the punishment would let B know it was A who did the reporting, hence to protect the "privacy" of A, the GM refuses to tell B the details of the infraction.
I've heard this happen in other games as well.
It's a dilemma. You can't expect someone who wants to comply in good faith to actually change anything if you don't say what is wrong, and you can't reveal someone who made the report in good faith. So when the situation only allows one and not the other, a GM probably has to choose to side with the reporter.