So far the map updates have been provided for free with the software updates that are also free. Navigon for iOS started like that too and then introduced a map subscription. Since TomTom owns TeleAtlas that provides the maps for their app I guess they can keep on providing that type of free update.
Yes data usage is really minimal and goes unnoticed with the current data plans, it would have been different on my first 5Mb GPRS data plan though in 2003 ;-)