Because you're not dealing with a community of fully rational people, who see a list of rules, and assume those rules are enforced as outlined. You're dealing with a community of real people, who will doubt if things are actually done the way they have been told if they never see them being done that way. Again, going back to the cop analogy, I don't need to know every last verdict and how every case turns out. I just want to pass some guy that got pulled over now and then, so I know that the person that cut me off and ran a red light I was going to stop at, isn't just the way things are around here.
Bringing that back to forum moderation, I don't need (or really have any strong desire) to know what action the mod took against the specific user. I just want the mod to post "alright guys, this thread has turned into person attack and isn't discussing balance anymore, I'm closing it". That way I know the mods care about thread quality, and I know that everyone else knows that personal attacks get threads closed.
In a perfect world simply having a clear list of rules would work, but the world isn't perfect, and the internet is less perfect than that. And in fact, even now people post and do things that are CLEARLY against the rules (porn, racism, etc) and other people don't even blink, because they think that is just how it is here. Visibility is about assuring Joe Q Forumgoer that those things really are against the rules, and the rules really are enforced.
Edit; Again, just to be clear. I don't think people want a compendium of moderator actions they can thumb through. I think they just want a quick explanation of what rule was broken (even if it was obvious) in the thread in question. To me that doesn't sound like a lot of extra work, but maybe I've got a bad frame of reference, since I clearly make long winded posts and I've got no idea how involved the back end of forum moderation is. As for the return on the time/effort investment, its an intangible. It comes back in the forum of generic board health/mentality. Ideally public action lets disgruntled posters angry about trolls come back from the brink and make good content, rather than pull an 'If you can't beat them, join them' response and start junk posting themselves.