At first I didn't understand what was going on, as its free content, however after a little surfing on the web, I found this post by Xbox's MajorNelson:
"Just a reminder that if you have a Silver account, you won’t have access to free content on the the Marketplace (like demos) when they are first posted. That content will be available to Gold level exclusively for one week, after which Silver members will have access to the free content.
The Fall update has a special message (above) to reflect this as well. If you have a Silver account and you see a piece of content that has a red circle with a slash through it (like the ‘Ryan Industries Plasmids and Gene Tonics’ content above) you’ll have to wait until a week after the content is released to access it.This is not done to annoy our Silver members—limited time exclusive access to content is just one of the many benefits of having a Gold Xbox Live account."
Now, Microsoft has restricted content to Silver Members before, providing some content exclusively to Gold Members either permanently or for a limited time, but they've never standardized it with all free content before, and Gold Member exclusive content has always been announced as Gold Member only prior to being released.
While waiting another week for free content isn't so bad, I'm certainly not going to shell out the cash for a Gold Level Membership just for this, and such a quick, unannounced policy change is a real slap in the face.
With the new BioShock content, for example, PC gamers get it right away in their patch, Xbox 360 gamers with a Gold Membership get it right away, and Silver Members get shafted.
Thank you Micro$oft.