So not only are you not thinking about the banned members' feelings, you intend on being nosy as well? Sorry for putting that in the rude way, but I'm a blunt person. I say nothing but the truth.
If everything from the staff forums became public to the community so that all would know, there wouldn't be a staff forum, wouldn't it? I'm afraid that there are some things that you need not to see, and that includes banned members' reasons.
Personally, I don't see the reason why it SHOULD be displayed. Even if it were, nine times out of ten you're going to be like "meh" and just move on. Nobody's going to be like "OH MY GOD HOLY -bleep- LET ME START A PETITION TO GET THIS MEMBER UNBANNED."
Whether it was posted or not shouldn't and most likely will not change the feel of the community, which is why in my opinion it should stay hidden, because it's not going to change anything anyway.