For the Apple TV - why are they not allowing you to "buy" media?
It doesn't necessarily mean the Apple TV needs storage - you should be allowed to buy media through Apple TV with all your media stored on the iTunes server and you can basically download / watch the stuff you have bought without paying a fee everytime and have it streamed on demand to your Apple TV.
I'm guessing the way around that limitation, for time being, is to purchase on another device or through iTunes itself. Steve said you can stream from your computer (without needing to sync) so you then should be able to access and play purchased content. I am curious if the streaming is limited to itunes content only or any network content. That would be a big blow to boxee and a huge benefit to me.