It is not really a rule. More of an observation of how trees grow. Branches grow towards the light and gravity pulls them down. In bonsai, the tree has to tell a story of how these two things drove its growth. Unfortunately, we do not have hundreds of years and a hundred feet for the tree to tell its story. We have to help the much smaller tree tell its story since gravity has less impact on the smaller tree in fewer years, we then have to hope that other folks will continue to help it tell its story after we are gone.
To be a bit melancholy, it is a bit like our relationship with pets. We outlive them and have to tell their story to the next generation. Trees will hopefully outlive us so they have to tell our story to the next generation.