There are reasons for the network's unease. For example, DVD sales of television shows have become an important part of overall revenue. Variety offered one example where NBC's Heroes DVD currently retails for $40 in stores.
Including 23 episodes, on iTunes it would currently cost $45.77 to purchase all of them. After the change, that cost would be $22.77, much cheaper than the DVD. Customers could opt for the cheaper download method, cannibalizing sales of the show on DVD.