Not only does Apple make higher margins on selling the hardware rather than capitalizing its cost, but the idea of a subscription service completely thwarts the strategic competitive advantage of their business model.
Subscription models already exist in the world of cable, dish and FIOS. Apple is seeking to undermine that old model with developing devices that can provide a truly interactive media experience in the living room: 100% a-la carte, on-demand programming, accessible through a user friendly interface. It's the interface that they're making money on, but they depend upon supporting this forward-thinking business model to give customers something the cable, satellite and FTTH companies will not.
Also, it's a philosophical matter. One reason Apple didn't simply give up on AppleTV in its initial failure is because Steve Jobs really, actually believes that this is at the heart of the future of technological convergence... the ability to harness the power of IP and the IP network to get the content we want, when we want, how we want, where we want. It's a revolution of the mind, more than anything... to change the way we think of entertainment and accessibility, technology, digital appliances and convergence. Why, for example, aren't all home entertainment components speaking IP today, which would allow for their coordinated management by one elegant, central system such as AppleTV?
Why in the internet age should we be locked down to paying a fixed fee to communally subsidize access to content we do not want when we can go buy an AppleTV and (with the forthcoming update) rent movies on our terms or watch tons of free podcasts on any subject under the sun without being beholden to any kind of monthly or yearly contract?
A subscription model is the complete antithesis of the kind of revolution Apple is trying to spark because it represents everything the revolution is against...