This is all quite fascinating considering that the DVD/Blu-ray release is still more than a week off.
What I find interesting about the whole thing is the fact that apparently this is a new norm for the release of movies. The only reason I found this out in the first place is because I'm getting ready to move and I packed up my Bluray player, but I bought The Cabin in the Woods on Bluray. It came with a code for a digital download and an Ultraviolet copy. Long story short, I didn't pack the Xbox and there is an "app" that you can download on the Xbox called Vudu where you can redeem your Ultraviolet copy of any film, which is what I did so I could watch my newly purchased movie. But, Vudu is also a rental service and you can buy movies directly from there.
There are several titles that are perhaps more "artsy" or considered "B movies", like Bachelorette with Kirsten Dunst and "Arbitrage" with Richard Gere that are actually still playing in theaters but you can also rent them through this service. However, for "major" films like DS, you can BUY the movie and watch it, but you cannot RENT it until October 2nd, which is the physical release date.
It seems as if they have some way of deciding whether or not to do this based on the films appeal and success. For example, "The Avengers" is not available for purchase, even though it will be released on the 25th. Perhaps because that was such a major box office smash the production company is building up tension so that upon release they can make the most money by making people wait for one of the biggest films of the year. For DS, however, it seems that the object is to make money however possible. And by making the film available early through digital download it's just possible that someone of a younger age who is curious about the film would buy it purely via impulse rather than wait for it to come out on October 2nd.