More a case of looking after your own interests, on Google's part.

If they gave everyone that kind of feature, they'd have no bargaining chip with big publishers. With YPN launched in the US, it'll be increasingly important to tie sites to using Adsense for the bulk, and not just in rotation.

Google could not allow major publishers to run 33% adsense, 33% ypn, and 33% of another network. They tie their premium publishers to continuing to use an enormous about of adspace for Google - otherwise they lose their premium features.