And in fact, when I buy a movie via iTunes, I have the choice of streaming it directly from Apple servers, or downloading it to my computer--or both. I generally choose to stream directly, because I'm lucky enough to have an internet connection that is both fast and reliable, and Apple's servers have been very reliable (not 100%, but close enough), so I don't have to use up my hard drive space.
But, if I choose to, I can have the full HD version on my local hard drive, and so can watch any movie I buy whether or not I have an internet connection.
(N.B.: I know I sound like an Apple fanboi (TM), but it's the only online service I've tried.)