What exactly is the basis for people thinking they automatically deserve any content the developer is finished with before release for the release price?
I'm curious.
It's not a matter of "deserving". If you make a game then sell parts of it piecemeal with a higher price/content ratio than for a full game, that's perceived as profiteering. Then there's the question of whether the full game was deliberately cut in order to make more profit, which amounts to the same perception.
The developers tend to say if it wasn't DLC, then the content wouldn't exist at all. While that may be true, we have no way to assess that claim and being suspicious of practices we can't assess by people who want to sell us something is, as I see it, a very rational attitude. Also, the only evidence I have (ME3's leaked script) appears to indicate that the suspicion is warranted.
All in all, if the publishers try to create a culture where selling game content piecemeal is more accepted, that, of course, is their right, but they shouldn't be surprised if their would-be customers complain about it and try to prevent them, especially if it's going to cost them more money. For story-driven games, there is the added problem that this may result in incomplete stories.