Personally I can kind of see both sides. On one hand, it does seem kind of bullshitty that you're buying a disc with X content on, but you're not allowed to use it until you buy it extra. It doesn't matter that you never expected that content to be included in the first place, it just seems like a dick move to not only finish some content before releasing the game but actually include it, but then not let the player have it until they pay some extra money.
On the other hand, I don't really understand the concept that if they made the content before release, they are obliged to include it with the game. At what point was the thing you paid for not just 'Game X', but 'everything the company has been working on between 200X and now'? Why shouldn't a company have the right to spend their time making whatever they want to make, and decide for themselves which of it to sell, which of it to give away for free, and which of it to lock away in a box and never show anyone?
On the other hand, I don't really understand the concept that if they made the content before release, they are obliged to include it with the game. At what point was the thing you paid for not just 'Game X', but 'everything the company has been working on between 200X and now'? Why shouldn't a company have the right to spend their time making whatever they want to make, and decide for themselves which of it to sell, which of it to give away for free, and which of it to lock away in a box and never show anyone?