I've actually had a purchased VOD title removed from me. I contacted Amazon to investigate and was told the the movie studio decided to pull the license from Amazon and go else where (probably an exclusive). I understand their desire for that type of licensing deal, however I had already purchased it and should still be able to watch my legally purchased license. What they should have done was after a certain date, you can no longer purchase it from Amazon but can still view your purchase. I complained about the "theft" of my video and Amazon was more than happy to refund my money. I don't know if it's a limitation of their system or fine print in the licensing but the government should step in and stop this practice.
That is why I've started bookmarking ALL of my VOD purchases just so I can keep track of them and know where it was purchased (Amazon, Vudu/Ultraviolet, Google Play, Target, etc.).