Why are there limits to the length of exposure when there is change or movement in the scene?

For two reasons, addressed in P4 and P5 of the Trust Test:

1. So as not to deceive the viewer about how rapidly any moving objects in the scene were actually moving (that's why the photo has to meet the “sasibe” standard if it is to meet P4/P5);

and

2. So as not to deceive the viewer about what things in the scene were visible simultaneously (that's why the photo has to meet the “satode” standard if it is to meet P4/P5)