A forum member has posed this problem...
LightSource.prj
That is the correct behavior. Only ray-traced lights can accurately account for the shadowing created by boolean cutters.
So, with the box as a boolean cutter, the sphere is completely "cut" and casts no shadow from the ray-traced light.
The z-buffered light can not handle that complex situation, so even though the box cuts the sphere from the camera's view, the z-buffered light still sees it and a shadow is cast.
Interesting way to get a shadow-only pass! I shall remember that!