They don't have to. In fact, they shouldn't give a shit how it works. To a consumer, that only thing that matters it the best use of their money. If one feels that announcing an Expansion Pass before the launch of a game is a bit of a pisser, then they can feel that way and wait for a GOTY edition (which their WILL be one now and we all know it).
The DLC would have been made anyway. It doesn't really matter if it's announced before or after the release of the game.
Anyway, if you call yourself a fan of games and like them to the point of discussing them on Neogaf and other forums while not knowing how gamedev works - I think something is wrong here. Or maybe I'm naive