I look at Steam as what iTunes should be. With Steam allowing me to download my games, I doubt Apple is suffering because of download costs. I'm pretty sure it's a holdover from their DRM-laden days.
For music and books Apple has to negotiate contracts with publishers who likely demand certain conditions in return for their content being made available on the various iTunes stores.
Considering Apple's consumer-friendly position on the repeated downloading of apps, it seems reasonable to guess that forcing consumers to pay to redownload books, music and videos is a requirement imposed on them by the content publishers.
Charging for re-downloads is just greedy, and smacks of the bad old Norton Antivirus days. The first time they pulled that on me, I decided I'd never buy one of their products again, and I didn't.