[Quoth]

GIFs simply as fixed palette alternative to png, no transparency or animation, though no idea why png without transparency not used too. Predates first kindle by 10 years. Perhaps a decision made by Mobipocket.

It's an electronic version of a printed book. Printed book files don't usually allow transparency or animation and white is regarded as transparent on paper books.

Thus if the file is printed on cream paper, the white areas are cream.