Amazon has 15 years of shopping history from me that includes multiple physical books each month and 0 kindle purchases. Its possible they don't consider that and default to kindle for everyone, but they at least have the info to know that kindle versions are not what I'm after.
Amazon is incredibly good at converting consumer surveillance data in to money. They could easily default this (and many other things) to sensible per-user values. Given their competence and attention to detail, the reasonable guess here is that playing dumb on this default pays better than doing right by the user.
Maybe what's happening is: if you have a Kindle (you bough it form Amazon - or you bought kindle books on your account), they assume is likely you want a kindle version. If you don't have a Kindle, they want to show you how cheaper the Kindle version is, and maybe you will end up buying a Kindle.
