I think IRL in the “rejection” case they don’t want to tell the user exactly why, since the user may be malicious and use it to try to evade the block. And for use in moderating UGC, well, most platforms don’t take seriously the idea that they need to answer to their users. Only their advertisers.

In the case of wondering why a bad thing got through, well, I think that’s why they just set these to the most pro-censorship level they can, to make that highly unlikely.