according to the square inverse law, it states that light fades directly in relation to how far you get from the light source.. at one foot, you're only getting half of the light as you would at the light source..
so, according to the square inverse law, if you chop off the leaves at the top of the plant, the leaves only one foot lower will only ever receive 1/2 the light as the leaves at the top of the canopy.. so, but cutting off the top leaves in an attempt to get more light at the bottom of the plant, at only one foot all that light is only going to be half as strong as it was for the leaves you just cut off... two feet and it's only 1/4 the light strength, so on and so on..
so to sum it up again.. the leaves at the top of the plant, the ones that are closest to the source of the light, are getting the strongest light for the entire plant, therefore doing the most work for the plant creating sugars and food via photosynthesis. simple raelly..