Downloading Movies - Availability Question

With TiVo you can download movies to rent from a number of vendors, including Netflix, Amazon and soon, Blockbuster. It’s a great system in general. One thing I noticed though, is that not all movies that are available for rent in the brick & mortar stores are available online. One specific example is The Pursuit of Happyness. It’s available to buy and download from Amazon, but not to rent. There are many other examples. Any idea why?

Presumably, making a video available for downloading requires different or amended licensing agreements, in addition to the agreements in place for hard copy rentals.

After all, a streaming download dramatically changes the number of people who have access to a movie, and the studios and copyright holders presumably want their cut of that increased audience.