US accounting rules sometimes require companies to charge for updates if they provide a significant feature left out of original products. An alternative solution can be to restate past earnings, something Apple would likely prefer to avoid.
Apple has sometimes been infamous for charging for updates typically expected for free. The most frequent example has been major iPod touch firmware upgrades, which cost a small fee up until iOS 4. In 2007, the company went so far as charging $2 to enable 802.11n Wi-Fi on some Macs.
