You call someone a consumerist because (replace "he, his, him" with "she, etc" where appropriate):
1. His tastes differ from yours. It is the same as saying "he is different from me, therefore he is an idiot."
2. You think he is acting under pressure from evil greedy corporations instead of acting in his own interest. This is just an anti-capitalist way of saying 1.
3. You think he is too keen on signaling his social status. By calling him a consumerist you are trying to signal your own social status.
4. You enjoy saying you are an idiot. This applies if you say "we are all consumerists." Perhaps you are not an idiot, but you like pretending so.