Great question, I am trying to get my head around this myself. I do think that the inverse square rule applies to what are effectively point sources. When light goes through a lens, it becomes basically a point source at the middle of the lens (I am sure that there is a correct word, which I do not know). When metering in a normal scene, you are measuring light that is coming from any number of very diffuse sources. I am sure that the inverse square rule applies, but that it is not measurable because there is not a single point to measure from.