As I pointed out in my response to Gabrielle's post about Facebook being "totally open" about the fact that user's posts and other content can be used in offsite ads or broadcast across the network without any compensation, the same is true for Kinja.
Someone could say "don't mainpage" all they want or restrict their stuff to just their personal blogs, but by posting or commenting to Kinja, you are granting Gawker Media license to do anything they want with your content. This includes licensing it whole or in part for use in ads.
These types of terms are pretty much standard for social media and the only real difference is that Facebook accepts users as young as thirteen, which is what prompted their "admission", while one must be eighteen to register for Kinja. This would be so that Gawker doesn't have to deal with the same age-related matters, Facebook and others have faced.