This is a feature not a bug. Heh Heh. Ya it *****. You can not purchase a movie to watch from iPad. You can only purchase the movie from the website. Here is a workaround. Follow exactly or you'll get looped around from app to website and back.
1. Open mobile web browser and go to amazon.com
*if your browser took you to amazon.com then go ahead and purchase a movie that will then be watchable from Amazon Prime Video
*if your phone opened the Amazon app then proceed to step #2
2. Don't touch anything in your Amazon app. Look at the top right hand corner of the screen and you will see amazon.com (normally where an icon of a cart would be). Click on it and your mobile web browser will take you toamazon.com. Now you can search for and Buy/rent movies that will then be watchable through Amazon Prime Video.
Hope this helped.
I posted this info on amazon support site:
http://tinyurl.com/hfn392z