One of the IMDB contingents for sale to Amazon was keeping it quite autonomous, and open. The database is still being exported to the same formatting as when it was traded on usenet; which is why one still gets files specific for Amiga and OS/2. http://www.imdb.com/interfaces
Kind regards
It's sort of asking - Why 'The New York Times' doesn't sell books. It does affect the choice but they don't set out to sell books. That is, sort of, what gives them authority. I might not trust the list as much if they were setting out to sell more and earn more.
If I go to, say:
http://www.imdb.com/title/tt0096928/
there is a "watch now on Amazon Video On Demand" link to the right of the page, rent from Blockbusters, buy from Amazon, etc etc. Even pre-acquisition they had "buy" links to the right, which I assumed were Amazon affiliate links.