The reason many people including myself may find this casting call to be offensive is because for a long time there has been deep and complicated issues with colorism within the African American community. Lighter complexioned African Americans females, especially those who are mixed race, have been generally considered to be "more attractive". Darker skinned African American females are considered to be "less attractive". There have been numerous studies done to show the issues of colorism with the black community and the physical and psychological effects it has on young girls and women. For example, there was a study done to show how darker skin is considered "ugly", where young black girls were given a white doll and a black doll. The girl wanted to play with the black doll and when she was asked why she chose the white doll, she said that the black doll was "black and ugly".
Millions of dollars are spent in the African American community every year on hair weaves and skin bleaching creams so that black women can fit the ideal standard of beauty that America has deemed as acceptable. Yes, they do have specific things they're looking for with their casting, but when many people read the description (and the apparent grading system that they have in place) it just reminds them that "perfect" women are those who look like models and their skin is light, and the ones who are less attractive are darker skinned.