I believe the cost has to be reasonable and in line to also avoid an accounting issue. Odds are, the $20 price is a preview of what future app bundles might cost as well for the iPhone and iPod Touch once the SDK is out. If they had priced it too low, it would have devalued future efforts that do demand proper revenue streams.

As for the AppleTV and iPhone, all have been sold under a "subscription model" based on the public information in Apple's financial filings and comments by the CFO. This was done with the specific intent to allow for free updates like these, and I think they are both spread out over 24 months. So instead of Apple getting $400 right off the bat in their financial reports for my iPhone, they report $50 every quarter. The iPod touch, as well as all other iPods, and computers the company sells are not sold under such a model, and hence the need for charging.