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 negotiate for unlimited downloads of music. This isn't the perennially-rumored iTunes subscription service, where you'd get a la carte music downloads for a monthly fee. Instead, iTunes music would have the same model as iOS apps: pay once, download as many times as you want. It's essentially the same cloud-based backup solution we heard rumors of last week.
It's not unlimited streaming or a subscription service, but unfettered downloads of music you've already bought is definitely a step in the right direction. According to Bloomberg's sources, Apple expects to reach an agreement with the major labels by midyear.