I doubt that there are laws that say you have to charge for updates.

BUT

Fortune is claiming that Apple has 70% of the MP3 player market share. The product is also closely linked to the iTunes Music Store. That means that Apple is in a unique market position, and Apple may be concerned that this will put them at risk of anti-trust prosecution.

I mean think about it: how would Linux and Mac users scream if Microsoft decided that the upgrade from XP to Vista would be free. It would be using their dominant market position to restrict access to competitors.