Because it's corporate fraud to hide sales results, or list them improperly on the books. SOx holds companies to much stricter accounting practices, and thus exactly what money came from where to fund what must be clearly documented.

Take these two examples:

John buys an iPod touch on October 1st for $299. This is during Q4. John bought a device that can play music, browse the web, and play videos. Apple puts this on the books as $299 income for Q4. January rolls around (Q1), Apple releases a free update, and it adds mail, weather and other features to John's iPod, features John didn't pay for, nor was told about existing prior to January. Apple has nothing on the books to represent this new feature for free, and thus it seems like an accounting error has occurred. Why? Well software development time was spent on these new features, adding cost to Apple for the product, however they did not properly account for this cost in the sale of the device on the books. Any preproduction research and development costs specifically attributed to the iPod Touch would have been on the books only up till the product was released in Q4. Any further research and development needs to be funded from somewhere. Bug fixes are exempt from this type of accounting, as it is considered post sale support, fixing problems similar to fixing defective products under warranty.

Now lets look at the same situation with the Apple TV instead. John buys it in Q4 for $299, and in Q1 Apple releases an update adding new features. This time, there isn't a problem with it, as the AppleTV revenue is not put on the books in Q4 as $299. Instead, Apple chose to spread the income reporting across a few quarters, thus meaning that the income from each sale in effect amortizes the software development costs after the product is released.