It prints what you tell it to print. If you don't want it to print a black background then obviously don't use a black background. If you told it to print a black background with white letters, and it didn't print the background but did print the white letters, then it would look like a picture of the Abominable Snowman during a snowstorm, and what would be the point of that? You control the output.
I thought the black background was intended to be easier on your eyes, not to have an actual black page with white letters in the final work... This way, if you have a dark mode with everything black, the pages document with a white background will be like a white Google page in the dark: blinding.
That’s why I thought the black background with white letters was sort of a dark mode for Pages, instead of an actual desired black page with white letters.