Disclaimer: This is not a proper theory in the slightest. It is very underdeveloped, and is not backed up by any quotes or WoBs. I am merely trying to get my thoughts out there, so that someone else can make a proper theory out of it.
It is my belief that the Simple Rules exist because the Shades of Therondy are attempting to prevent people from accidentally tapping into the latent investiture of the shard Ambition, or trying to prevent large concentrations of investiture gathering in one spot. To review, the simple rules are: Don't kindle flame, don't shed the blood of another, don't run at night.
Flame is the essence of change. By lighting a fire, you could be seen as making a statement that things aren't fine the way they are, and must be changed. Change is also a fundamental part of Ambition, as attempting a change is saying that your vision of the world is superior to what already exists.
Running at night invokes the idea that your destination is the most important and that you have little regard for the way things currently are. When you run, all the disruptions you make from movement are greater. You disturb more earth, your footsteps are louder, and you move with less caution. These are all statements of ambition.
Spilling blood should be pretty obvious. When you spill the blood of another, you are making the statement that your life is more valuable than the one of the person you're killing. That is indeed a very ambitious statement.
In conclusion, the Simple Rules exist to prevent statements of ambition, and it is my belief that they exist to prevent people from tapping into the investiture of the shard. What do you people think? Any assistance is supporting or debunking this would be appreciated.