WTH? If the sun (Light source) remains in the same place all the time, so will the environment shadows... I'm not sure what you're arguing... The only difference visually between what DC is doing is that it doesn't have day/night transitions, the sun remains in the same place, therefore the shadows cast by objects hit by the sun will remain where they are, after all the sun never moved, duh. The only thing that matters is the perspective of the driver, inside his car the sun is moving, the reflections are moving the shadows are moving, all dynamically and in real time.
Like taking an abject and moving it around a lamp. Rotate the object, turn it, spin it and the lighting/shadows will behave accordingly and cast around the room and into and around the object. The existing shadows from other objects in the room will stay where they are unless you move the lamp.
Visually it's exactly the same effect, the only thing different is the moving light source which is needed for day/night transitions, then the shadows in the environment have to be moved dynamically or it would look ridiculous, but since it's not moving in Forza, it looks the way it should, so please stop arguing semantics.