Before we begin, some background is helpful. How do the researches intent "to the test whether employment websites discriminate based on race and gender"?
They plan to use what is called audit testing. Researchers pretend to be people of various races and genders applying for employment to see if the websites treat different races and genders differently. Audit testing is widely used offline.
The problem is in the pretend to be people of various races and genders. Facebook's Terms of Use for example says
https://www.facebook.com/legal/terms
The researchers cannot comply with this provision and conduct their research.