Why does TTG put no limits on “light”-related changes except meeting P7?

Because viewers are less distrustful of “changes that always happen in photographs.”

Some “light”-related aspects (tones and colors) always change whenever a photo is converted to a different medium (e.g., from screen to print) even when no one intervenes to change them.

Why does TTG severely limit all non-“light”-related changes?

Because viewers are more distrustful of “changes that never happen unless someone intervenes to make them.”

Non-“light”-related aspects (forms and shapes) never change when a photo is converted to a different medium unless someone intervenes to change them.