That said, once every month Apple releases a new free indie pack for download, and even I, for whom indie is not a thang, download it and get exposed to twenty new bands I've never heard of before.
That's great that "once" a month Apple releases a new indie pack. As if that truly compares to the amount of heavy rotation that major artists receive on iTunes. Not. Sorry to tell you, but that indie pack with 20 new bands once per month, is not getting anybody any significant exposure, nor helping any unsigned musicians to really earn a living from their music. Especially not in comparison to the millions Maroon 5 and others are getting.
Lets take notes from YouTube and Digg.
My point is that Apple's aware of that and does what it can, little as that might be. They use indie music in most of their commercials, promoting an artist at a time (though never anything edgy), they offer free promos of musicians, though not many. That's all they can do. There are thousands of indie artists, Apple can't show them all off, and to be honest most people don't care about them. They care about Maroon 5, because Maroon 5 is easier to swallow.
iTunes isn't trying to make a statement. It's trying to reflect popular opinion. I wouldn't doubt Apple would be glad to be able to show off something more diverse, but our culture isn't diverse and so Apple's not going to do a thing until that changes.