As ex B/U mod, head mod and admin and as one who made and reviewed the rules, I think I can give you an answer.
1) Admin is a respected and very experienced person who is trusted enough to be given powers to ban.
More precisely, anyone from B/U crew + admins + some other ppl can issue a ban - either by solving ban request or by banning you "straight away".
These ppl are trained enough to issue right/proper bans. As a guy who was in B/U for years, I can honestly tell you that after some, quite short time you feel nothing while banning. No joy, no happines and no satisfaction.
Mod just issues a ban and moves on. That's all.
Also a mod who was in your game knows the situation better than someone else, someone who would spend max 10 mins reviewing it.
This is why nobody else reviews the case.
2) This place isn't democracy, never was, never will. The one with most powers rules. You can imagine him as a dictator.
That's why admins words are always indisputable.
If he says that your ass smells, your ass just smells. Deal with it or gtfo. There are no more options.
3) About that admin's above the rules... Only top admins are above rules, or at least that's what I've been told.
Section admins are more like "Supreme mods" (more than heads, less than actual admins).
As I was told, these top admins did so much for this place, worked so hard and long that they deserve full power to do anything they like and nobody can do anything.
I actually argued about this matter with MNG while I was B/U admin.
I believe rules should apply for everyone in spite of power or hierarchy position. Why can MNG ruin your game? Why can cen insult you based on your religion? Why can MUN abuse a share?
Answer: Because they did a lot for this community.
I'm not saying they did, do or will do that, don't get me wrong. But if they want, they can do it. Deal with it.