Apples and oranges again! The percentage of American TV owners with cable or its equivalent is huge. The eReader market, by comparison, is still very small. Also, the experience of seeing movies at a movie theater and on a TV are utterly different, while that of of reading an ebook and reading a print book are really very similar. But even more importantly, the money to be made from a movie going onto cable is not that uniquely linked to who watches it on cable. If a publisher holds back the ebook version for a significant length of time, he runs the risk of eReader owners simply forgetting about the book. If it's no longer being reviewer or talked about, what will prompt the eReader owner to buy that book?
Obviously, the publisher is hoping the eReader owner will buy the hardback. A good percentage of the time, that's simply not going to happen. Having an eReader is a little like having an ice maker. Once you own a fridge with an ice maker, going back to one with just ice trays is a royal pain. For me, hardback books are ice trays.
Add in the fact that the people most eager to buy ebooks are voracious readers and you have a recipe for publishing disaster.