When you buy an app for your iOS device, you only have to buy it once. If you buy it on your iPhone, you can download it on your iPad later and pay nothing at all. If your Mac's hard drive dies on you, as far as your iOS apps are concerned it doesn't matter; you can download them again later without paying a cent.
So why isn't the same true of music downloads from the iTunes Store?
According to Bloomberg, Apple's recently been asking that same question of the music labels and trying to
