Two reasons
1. a shaved white woman is like an anatomical model, making it easier to see her genitalia.
2. white is the traditional colour of innocence, so a white woman being fucked represents a corruption of innocence and maybe a touch of misogyny thrown in, depending on context.
Mainly #1 though.
No. If you look at e.g. the movies shot by Mason at HardX, you will see the darkest of asses and pussies shot in great and revealing detail. She shoots a lot of black girls having anal sex with white men, and she does it well. If anyone is looking for a tip.