Discussed plenty of times here on UKE and on saafe.
In some cases it's not wanting to meet relatives or acquaintances.
But there are black girls who show their faces (so aren't afraid of being outed) and will still not see black guys.
It's a combination of preferences and prejudices.
Escorts have more to lose than punters if they accept a booking from a weird, violent, annoying or stalker punter.
Say you have noticed that a third of punters from certain groups are too annoying (be it haggling, boundary pushing, being violent, being too big or whatever). Yes, it is a racist generalisation to exclude all punters of that group, but I can understand it. It's not different from punters who refuse to see WGs of certain nationalities because they associate them with bad service.
The key thing to bear in mind is how much WGs have to lose. If you are hiring someone you may be willing to overcome your prejudices and interviewing someone extensively. If you're a WG and accept a booking from a dodgy or annoying client, it may end badly.