Why do commercial chocolate/cocoa products often contain fat-reduced cocoa?
Without getting arrested for loitering in the baking aisle of my local supermarket comparing weights, ingredients and prices, to me it is all down to price and processing method. The low starch, natural fat product performs better according to Cooks Illustrated.

Higher natural fat = premium product = better taste. 

References:

Alice Medrich: https://food52.com/blog/9521-how-to-identify-good-cocoa-powder-and-not-so-good

Cooks illustrated: https://www.cooksillustrated.com/articles/663-which-cocoa-powder-should-you-buy