For me, the argument isnt with people buying the DLC. I don't mind them. The argument is with 2K charging a hefty price for it. Now, if this was the only DLC, that would be one thing, but they will probably have more released ... say they add 3 sets of DLC, that's $22.50 (price ~ $7.50) for six civs and three scenarios, assuming it follows the current model.
As a customer, I have a right to demand what I'm paying for. Consumers should be advocates for themselves, otherwise they are liable to spend money unwisely.
Compare this to BTS or Warlords. Yes you paid more money, but you got far more total content for the money you paid. You have new objectives (Apostolic palace victory), new wonders, new civs, scenarios, people, buildings, leaderheads, units, more strategic depths with vassalage, etc. And the game itself already initially came in a more "complete" manner, with all sorts of elements which aren't in the current game, like religion.