This has nothing at all to do with contracts, carriers or monthly payments to your provider.
With both the AppleTV and the iPhone, For accounting reasons, Apple decided to recognise the income in a different way; as a series of payments instead of one big lump.
What I am suggesting is that once the device is really old (24 months +) 100% of the revenue of a product will have been recognised, Apple will no longer be able to treat the device as if it is within a paid subscription.
The accounting rules have it that once a consumer has bought a device, they can't have free new features without a payment. Anyone remember the 802.11n "enabler"?
The alternative is simply that after 2 years, Apple simply stop updating and only offer free bug-fixes.
Either way, it is no big deal, the likely cost of a paid feature update is probably going to be $5 or $10.
C.