The idea behind digital copy is that it allows people to have the HD experience on the big screen with the disc, and also the on the go experience with a portable version. By keeping the resolution down at normal DVD sizes, and combine with H.264 compression, they can keep file size down to make it portable friendly. If they made the digital copy full HD, then they run the risk of it not playing on a portable. It would also have a much larger file size, potentially requiring a second BluRay disc just to hold it, and thus requiring a BluRay drive in the PC to make use of it.