Three firms control 89% of US soft drink sales . This dominance is obscured from us by the appearance of numerous choices on retailer shelves. Steve Hannaford refers to this as "pseudovariety," or the illusion of diversity, concealing a lack of real choice.
If you assume that people have the same values as the author and want to establish their class credentials by buying from a minor player, then it's proper to conclude that the only reason they buy from the big three is that they have no other choice. If you assume that most people follow their friends' tastes, are responsive to advertising, and don't care much about the minor differences in taste between one sugary drink and another of the same kind, then you can restore common sense and say yes, if there's a beverage on a shelf in a store with a price on it, then anyone who walks by it has a real, non-illusory choice between that beverage and another one. They drink Coke and Dr Pepper because their social norms don't oblige them to take a symbolic stand against the financial dominance and boring aesthetics of major corporations. Their desires and norms tell them to drink a mainstream, ubiquitous brand.