Sure, you can use your netbook to consume all of this content as well, but not (yet) with a one-click "buy now" experience. Just look at Amazons success with the Kindle, and this conclusion is obvious.
Previous attempts like the UMPC was trying to take one use case (desktop computing) and shoehorning it into another form factor. Apple will most likely be trying to expand the iPhone experience to a bigger device. They will have a much higher probability of success with this move.