My guess on Apple's reason not enable it is just to avoid user confusion.
Since the iPod Touch and iPhone are able to view PDFs, read e-mails, movies, and everything else, users might think dropping a PDF onto their disk-enabled iPhone would be viewable on the device itself.
But I don't know, it's the only real reason I could think of. And I know there is a counter-argument about iPod Nanos and disk-mode but I'm just saiyng.