iPad has Amazon app.
If I see a book on Amazon that I'd like to buy in Kindle format, "this mobile application does not currently support digital downloads. Add this item to your Wish List and you can view and purchase it from your computer."
...and I'm just left there staring open-mouthed at such complete, utter techfail. The wonders of a life lived on the move through one useful device completely upended in two brief sentences. It's just basic connectivity, guys!
(OTOH, this has to qualify as very much a First World Problem...)