For me, it all really boils down to a simple fact:
Quite a few people are unsatisfied with Blizzard's performance regarding some areas of the game (content design, quality, longevity etc.). Thus, I have good reasons to believe that subscription money mainly funds projects X and Y and not WoW. With cash shop it gets even worse - I have no reason to believe that subscription money goes anywhere else than funding Project X and Project Y *as well as* making more stuff for cash shop! For me that's an unacceptable practice and that's why while I'm perfectly OK with sub-based game, I'll never support game that double (triple-, quadruple-) dips.
In my opinion, most people defending such 'corporate business decisions' are very shortsighted and unable to connect the dots.