It has nothing to do with raytrace shadows.  Without getting into the specific scientific explanation, the inverse square law pertaining to lighting is a falloff of illumination with distance from a light source.  This can easily be demonstrated in Poser by placing a point light in a scene and changing its attenuation from Constant to Inverse Square.  Below are 2 renders: the first is a light source with constant attenuation at an intensity of 100%.

The second render is the same light source at the same intensity but with its attenuation set to Inverse Square.

As you can see, there is significant light falloff with distance in the second render, this is the typical behavior of realistic lighting.