Because “light” is the root of all photography...
There are two ways to change any photo
1. “Light” related changes (tones and colors)
TTG puts only one limit on the #1 changes: the combined result cannot misrepresent the appearance of the scene (see P7).
2. Non-“light” related changes (forms and shapes)
TTG severely limits the #2 changes: the only allowable changes are listed in #1-9 here.