One place Amazon appears to be noticing the benefit to reduced or eliminating DRM is in game sales. On Reddit in the /r/gamedeals subreddit, we often discuss if a game has DRM before concluding if it's a good purchase or not. The Amazon rep Tony often hangs around there to answer questions or posts deals himself, and he sees that DRM-free games do sell more. He now says if a game is DRM free or not when posting deals.

I understand they have also had a few experiments into working with publishers to sell games without DRM to measure the effect.

So, just an anecdote from a small part of Amazon, but that information may propagate to the books section eventually. We can only hope, because nothing puts me off purchasing media like digital rights management.