Same way it is up to me how you use books you bought (kindle) or songs you bought (any streaming crap nowadays).
If you're shitty enough person (good businessman) you can profit off of anything.
If you're shitty enough person (good businessman) you can profit off of anything.
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.