What they have done to discourage people from making the files available all over the internet is that the copy you get is personalized. They mark the file with the name on the credit card used to pay for the download. Both as a three second tag in the beginning of the movie and as an invisible watermark through the file.
I guess it is possible to remove this watermark just like it is possible to remove most known types of DRM, but I think this approach is way better. And very user friendly. And future proof. The MPG4-files that you download will play on Mac, Windows and Linux through free tools like the VLC Media Player or Democracy Player.