It depends mainly on how Apple defines "major new features" as opposed to bug fixes and minor refinements.
Bug fixes and minor refinements are fine to give away for free because they don't constitute any significant change in functionality from the product they originally sold to you.
On the other hand, if they add major new features, according to generally accepted accounting principles, apparently they technically sold you an "incomplete" product in the first place, and they're making the product "complete" now by way of the firmware update. In order to do that, apparently they are not permitted to put their complete revenue for the sale of the product on the books until after the feature-complete product has been fully delivered.
Since Apple apparently puts the revenue for the sale of an iPod touch on their books immediately, the product is deemed to be complete upon delivery and no major new features are permitted. To get around that limitation, they can designate the the major new features as a separate product, distinct from the product they initially sold to you. But in order to do that, they apparently need a separate line of income on the books to cover those new features.
They don't need to do that for the iPhone, apparently, because they break down recognition of the money they've received for the sale of iPhones over the course of several years. (In reality, only the original iPhone 2G involved a real ongoing flow of revenue form the carrier to Apple over the course of the iPhone's contract. But apparently there is some accounting magic they can play with subsequent models of iPhones to have the same effect on the books, even if there is no real flow of dollars. They didn't use similar accounting magic with the iPod touch.) Hence the delivery of the feature-complete product can be broken out over time as well.
I'm sure there are other subtleties that are missing from my analysis, because corporate accounting seems horrendously complicated and I am not competent to understand all of it. But I hope I've got a reasonable understanding of the core issue.