This is a strange move on Amazon’s part, because there does not seem to be a technical reason to wait for the SD version. It looks like Amazon is trying to just get more money by forcing anyone who wants to watch select new movies to pay for the HD version even if your Internet speed or device will not stream it in HD.
This is also a detraction to anyone trying to cram more movies on a tablet with limited space before a flight. A SD movie takes up considerably less space than HD movies, allowing more hours of entertainment.
This may not be something that Amazon is doing. In the end the rights to SD and HD versions of a movie are owned by the content creators. Movie studios may be behind this move to release HD and SD versions of new movies at different times.
This is not the first change in the least year to how Amazon sells content on Amazon Instant. Last fall Amazon changed how season pass works, forcing you to pay upfront for a season if the content creators wanted to set it up that way. They also no longer credit buyers who already purchased a few episodes to try out a new show before buying the season pass. The price of the pass stayed the same no matter if you already owned 3 episodes of the season or none.
We have reached out to Amazon for comment on how widespread this will be and will update this story when we hear back from them.