First of all, iTunes DRM is not designed for a subscription model. Re-engineering would be required, including firmware updates for older iPods, to enforce the subscriptions.
Moreover, not all songs are typically available via the subscription model. Jobs continues to make an issue about variable pricing for songs, with the DRM-free option being the one exception. Yet, consider how they are planning to implement this: by a preference in which the user selects which kind of music s/he prefers to buy.
Some have said a subscription model would require a whole new iTunes Store -- a separate store, with rentable tracks. This is not really true -- users could be presented with a "Buy Song" or "Rent Song" button where applicable.
A subscription service is "not out of the question," he says, but it doesn't look like it's in Apple's interests -- they would bear the price of increasing download costs, unlike the record companies.
DRM-free music, on the other hand, allows for seamlessness. Users can download music, copy it between iPods, computers, and friends' computers without a hassle. Rentable tracks would lend themselves to the opposite kind of experience.