This whole thing is interesting to me. I think we're sort of watching a paradigm shift in the way publishers and consumers interact.
Just isolating at the economics of it, why does it being on the disc matter? Everyone who purchased a copy of the game knew what they were getting into. They willingly exchanged money for the game as it was. This unlock was not included in that transaction. Then, the publisher asks people to pay more money for additional content. People decide whether or not they want that content.
However, we have this notion that once we've bought a 'thing' we should have full access to it. I like this idea, personally. I think most of us here do. When they reveal that you bought the disc, and it had the content ready to go and you are locked out, that's evil.
However, if they did the same thing but shipped it without this content on the disc, that would be OK? If they COULD have put it on the disc, but they didn't - does the publisher have an obligation to release the content if it is finished? I think that gets a bit more gray.
What if they finished this the week after the disc shipped? Is that OK?
Is it that we're theoretically 'covering the cost' of the development of the game with our $60 or $50? And then the price of DLC is an incentive for them to continue expanding the game? On the other hand, they delivered a game in a state that you can choose to buy or not. What is hidden in the disc's dead space is of little concern, right?
DLC has caused some interesting ethical and financial quandries. One the one hand, it seems like game prices are going up by degrees. We're paying $60 for a game, PLUS another $5 here and $10 there. Some games, especially multiplayer titles, may cost you upwards of $100 by the time you're finished. Are we getting our money's worth? Are we getting a good deal for our gaming value? At the same time, do publishers have an obligation to tell us up front what we're getting into: ie, you will pay $60 for this game and an estimated $x/interval for DLC in order to have the 'complete' experience.
Not to mention the whole 'project $10' initiative - where there's a code in the box that you can only use once, and it locks used owners out of content that you would otherwise have to pay for as DLC...
Complicating matters is that there's not any competition in the market - if you want a COD:MW2 map, for instance, you're getting it from IW/Activision/MS Live. There's not a competitor that can sell you a similar product at a competitive price.
I think the future is going to be full of more of these practices. And, by and large, the average gamer is going to be oblivious.