They could make a base image and then an image layer that contains the game files only. Now you have a single file image per game with a consistent base.
My point is that by separating the layers it does fulfill the declared goals but in a more portable way. As implemented, there could be hidden dependencies on the local file system or across game installs.
I guess one concern might be the author wanting to push the images to a public repo so other people can use them - which would probably get the author into a bit of trouble for distributing paid content.
