Greetings members new, young and old! I have come out of the woodworks to attempt a satisfactory response.
I'd like to extend my sincere apologies for disappearing for so long, and consequently would like to thank Tom, who picked up the pieces I left behind, and the rest of the community for not giving me any heat whatsoever (yet..

) for leaving without a trace. That being said, I would be completely fine to be relieved of the Staff duties assigned to me, not that I actually substantiated much work outside of writing up a few things and assisting with Q&A on the boards.
I will say a few things on the then-established rules, and the current situation whilst spilling some of my personal opinion.
Those who know me and my posting habits might know where my post is headed, so before I commence, I'd like to clarify to Phantomganon (I'll be referring to most people by their board names, since they've changed to the point where someone who has been absent for some three years would not recognise.. I humbly ask that you do not feel offended.) that you asked for reasons, and should it be that my post is a bit lengthy for clarification, I hope this is not a deterrent.
First, we need to concede that MKSC is the Mario Kart game with the most strategy variety and possibilities.
Concede it with me, please!
There is a multitude of solutions for many tracks, some of which demonstrate more severe shortcutting tendencies than others. With such a wide range of diverse approaches to driving from point A to point B, some rules were in order. As others have noted before me, and I'm sure in my absence, (Thank You!) the key to the rules is to keep the driving along the lines of Non-SC.
The object has not been to establish rules for any given bumper, gap, offroad surface or other element that may compromise this, or lead to Non-SC validity speculation.
We attempted to compile a set of rules that would primarily eliminate SC driving. In the process of doing so, we did throw a few conventional routes out the window. Routes and driving lines that anyone would find and use. RiRo comes to mind. This may seem arbitrary or "random" as some of you seem to prefer labelling it, but it was all done to favour a more Non-SC approach for this particular track. The conditions and designs for each track are so wildly different that it does not make sense to establish one set of rules across the entire spread. A few fundamental and not-so-intuitive techniques and actions were still specified as general, as they are universally banned. To me, it would make a lot less sense to ban for instance "all of the bumper jumps in BP and SG" as opposed to those that most prominently affect the Non-SC driving.
Would you rather have a set of rules that say, banned shrooming across the water in all places applicable? Wouldn't most agree that jumping across the water in KB1 to circle the finish line would be more of a shortcut than jumping over a portion before the home stretch? I think most would agree the first example should be deemed a shortcut.
This is largely why we see a track-by-track breakdown of the rules.
Then again, what's Non-SC is debatable, but I consider the rules profound and reasonable to the point that they are understandable from a Non-SC perspective. You might not like all of the choices, but I think it should be fairly clear why some of the strategies on BP for example, are considered abusive. It's perhaps easy to feel we should be more lenient towards courses that so clearly invite jumping and bouncing, but ask yourselves whether it truly is in the Non-SC spirit to consistently traverse barriers airborne, or knowingly drive backwards.
What constitutes "cutting too much track" is hard to define, and will be arbitrary to some extent, but most decisions made appear fairly sound to me.
I'll also briefly explain the fuzzy wording on YD. Whenever I hit the bumper on YD I would swerve
around the corners in mid-air so as to make it feel Non-SC. The only benefit would then be the additional speed and travel distance of hitting the bumper with a shroom.
You really can't see whether you clipped some barrier mid-air, (hence the wording) but I did
not so much think - to the detriment of my own times - "Wahey, this is a green light to go straight over." I can see that others might consider this fair game. As for the YD cuts presented, I would personally be in favour of allowing the first shroom. But I'm not so sure it would be easy or appropriate to introduce the possibility now.
No one has ever alone decided the rules or any part thereof. My singlehanded responsibility and input towards the rules now in place is not greater than anyone else's. I took it upon myself to write the document back in the day. Surely, there are things that I could have worded more elegantly and concisely, but for the purpose of outlining accepted routes, I think it works fine.
As for the AB-spin technique that has surfaced since, my personal opinion has been very well
presented by Etch. Thank You. It really does not fall within conventional Non-SC driving and also serves to prolong the boosts. Is a lengthy discussion really necessary, knowing that ticking is banned altogether? How much time saving potential it has, how difficult it is to perform, how fun it is to perform, how fun it is to do(!) and whatnot should not be a factor in its acceptability.
Andreas