Even though you technically own all of those ones and zeros for the extra content that are on the disc, you do not own the license to play them until you pay the additional $15. You own the disc (and the 1s and 0s) but the creator still retains control.
On this we agree?
I'll start out by pointing out that this has nothing to do with the original conversation. To answer your question, I don't know for sure. Clearly you own the material, in as much as you own the right to unlock it. Whether you could circumvent the protections to access the content for free . . . I dunno if that's ever been answered, but I don't think it's an open and closed case. Obviously a person couldn't sell you a house, intending to tell you later that hidden in the wall was a valuable treasure but you'd have to pay him more for him to tell you where it is. I mean . . . he could do this, of course, but if you opened the wall yourself and found it, it's yours. The DMCA gives me pause with its prohibitions of circumventing copy protection, but I don't know if they'd consider this copy protection, so I don't know if the DMCA applies. I mean . . . I don't really know the DMCA at all, frankly.
So, yeah, if the extra content is on the disc and you manage to access it without paying the company more . . . I don't know. I tend to think that the customer would end up winning, but that's just my gut talking. As far as I know that hasn't been answered difinitively, and if it has it may have gone the other way.
God knows Take Two and Rockstar paid out the ass for Hot Coffee, even though you actually had to modify your console, and then modify the software in order to get at it. If that's all we have to go on I'd tend to consider everything on the disc as owned and available to the consumer, even if the developer/publisher did not intend it to be.