To clarify, this happened because NBC doesn't own the distribution rights to all the shows they broadcast. In the world of television, networks and production studios are separate entities. It's not uncommon for a studio such as NBC to produce a show that will air on another network like ABC or Fox. And generally, the production studios get the final say as to how a show is used after it airs on television (i.e. DVDs, syndication, etc...). In this case NBC didn't get the say on all its shows on the iTunes Music Store. That explains why some shows, such as Chuck (pictured above and produced by Warner Bros.) are still on iTunes.

It also seems only this season of NBC shows have been pulled. Prior seasons of NBC shows previously sold are still available in their entirety. [setteB.IT]