Amazon and IMDb websites are only sometimes linked when a title is watchable on Amazon, and someone contributes the link on IMDb. Amazon extracts information from IMDb to display on an Amazon page, but notice they don't even provide a link back to IMDb. The sites are predominantly independent.
I'm not at all clear what culling you are expecting Amazon to do. Keep track of titles you've rated on IMDb? You've watched on IMDbTV? Cull them from what display? I don't even think it would be a good marketing idea, because people like to re-watch some titles. Amazon doesn't even remove something from your Amazon Watchlist just because you watched it. What if you'd actually fallen asleep, and actually missed part of it?
