So by your logic, just because they don't provide all available forms of transparency means they don't provide any? You are just searching for a "gotcha" when it isn't even a counterpoint.
As to why,
If Mbin cannot support this then private communities need to block Mbin instances completely.
That particular exception exists, it doesn't take too much effort to find out why: https://github.com/MbinOrg/mbin/issues/1115 Among other things, Mbin was threatened to be blocked by Lemmy on default for part of its functionality by the Lemmy devs, and melroy decided to succumb to the pressure rather than split the fediverse on this issue. Honestly, I find the main Lemmy devs to be compromised given how much they want narratives to head in a particular political direction and the tool that lack of transparency on downvotes provides for that in regards to shaping visibility and discussion within threads. It wasn't like that for a long time, but you knew that.