Their white customers dont like them entertaining black men.
Its a reflection of the porn industry where interracial isnt supported by racist white customers either.
To appeal to their racist white customers they say no black men thus getting more rich white costumers.
Its all about getting the most money and with the desperation they have which drives them into doing prostitution in the first place they have no problem going the extra mile doing that so they can get more rich racist white costumers who'd like nothing more than a reminiscent from the good ol days where they had white only sections.