The real reason is that the movie studios want to control when to release movies into circulation through different channels on each continent.
Usually, a movie first comes out in theaters in the US, then in Europe, and then in the more backwater continents, such as Africa, Asia, and Australia.
This creates a situation whereby the US DVD is already out while the movie is playing in theaters of other continents. If people from other continents could import the US DVD and play it easily in their homes, they would not bother going to the theaters. That would cut into a very lucrative revenue stream for the movie studios.
However, since refusing to export DVD's would break international trade treaties, the movie studios have decided to break the law using a technological solution. They are currently under investigation for this by the Australian government.