DRM on books is about the rights holder, not the person selling it. I can assure you that Amazon doesn’t actually give a shit what you do with your books outside of having an obligation to the rights holders.
Yeah, because they wouldn’t be able to offer it otherwise. If they could offer the same content without DRM at the same price, I’m certain they would do it. Apple did with the iTunes Store.
They wouldn’t, simply because their position as the dominant player in ePublishing gives them a massive incentive to lock-in users. Which is what their DRM does: you cannot move books to non-Amazon devices, so you’re stuck on Kindle forever. That this also satisfies publishers is something of a side-effect.
They did the opposite with music (they actually sold simple standard mp3s, unlike what you get from iTunes) because they were challengers, not incumbents, so the priority was to attract users.
I assure you that they care much more about you buying books from them than about "lock-in" from you using their apps and buying their devices. Proof: their apps absolutely suck and their devices are not much better. The devices exist to get you to buy more books from them. It is absolutely not a side-effect that their DRM satisfies publishers.