Seems like maybe iBooks was intended to be a "first class" app but they couldn't make the schedule to make the OS image, so they decided to distribute it over iTunes.
And perhaps they wanted to allow Kindle etc on the store without raising the controversy of why they don't block them since they duplicate built-in functionality.
I would sure want the luxury of instantly pushing out a minor update to all users of the app rather than having to wait months if I was internally developing a first party app.
And perhaps they wanted to allow Kindle etc on the store without raising the controversy of why they don't block them since they duplicate built-in functionality.