As I understand it, that capitalism generally implies that "white" is not being used specifically to mean something like "the name of a group of a specific group of people from a given region/culture", instead it is being used to mean the default American culture that provides the frame of reference against which our society tends to measure other cultures. So for example, "Black" would be capitalized because it is a specific group separate from the default of "white" while "Anglo Saxon" would be capitalized because it is a specific subgroup within "white".
This probably isn't a perfect analogy, but it might be somewhat similar to not capitalizing "earth science" but capitalizing "Lunar Science". It's assumed that if you don't specify then the default is that the science you are talking about is on Earth.