Obviously the term "tax" was metaphorical - to scoff at this much was trivial. Call it "Progressive Reduction in Revenue." My understanding is that the scale in "progressive" is the popularity of the author (and thus their average income... as in a progressive tax).
What proof do you have that this is a bell curve, in which those authors in the middle lose the most money, rather than those at the top? I find your observation highly illogical as it only makes sense the most popular authors would sell content that is mostly dearly pursued (and so, most likely to be pursued until it is obtained for free).