The Fountain already has a bluray release you can buy for below $10! Why would Criterion release it and why would you buy it?

I believe Criterion's goal is to release films in a quality edition that would otherwise not receive one or may not get a release at all.

You can tell my bias on this issue from my Criterion list on the old forum - the large portion of Criterion films I own are from other labels who also did an excellent release (eg. The four Fassbinder sets from the UK where you get twenty-five movies for the price of the Criterion BRD trilogy and Fear Eats the Soul. There is no difference in the quality as both sources come from the Fassbinder Foundation.).