You didn't buy the DLC on the disc. The disc is a key to access online software. You paid for the right to access the software that the developer owns. As it is their software they reserve the right to allow you access to what they see fit.
This is not 1985 when you buy a Nintendo cartridge the game is yours.
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Agreed. My point is simple though. And you are correct. IP laws protect the business. And I protect my money. ;o)