"We're talking about transfers between platforms, not transfers between devices on the same platform. The latter is trivial in all cases, even the iPhone" you might be making that point but we are not! We are talking about transferring your content between the device you have now and the device you might need in the future, if you put your eggs in apple's basket you are fixed to their (limited) vision of your future, which maximizes their profit margins, however if you put them in the android basket there is a better chance that a provider will produce something that fits into your future and in that case you can transfer your things with no penalties.
your point about kindle would be ok if kindle didn't provide an app for all of the major platforms or apple provided an ebook app to enable you to read your books on other platforms.
and of course the last defence of the cult "they're not the only ones doing it" i.e. its ok that we behave badly because everyone else does too.. I disappear!
Ultimately if you join a cult you have to be prepared to pay the exit fee.