As has been stated ad nauseam, the thrill for the whites is the taboo. Many of these people look down on american blacks and encourage other groups and nationalities to despise them too. They don't behave this way to other groups, just the american blacks.
Many whites also take delight in ridiculing and making fun of the blacks, and encourage other groups and nationalities to do the same. They don't behave this way to other groups, just the american blacks.
Whites also don't need to hide behind a lifestyle when socializing, fucking around with or being more involved with people from other groups or nationalities. They do this only for the blacks. So coming to hide behind a lifestyle and for your husband to select one of these very people they despise and ridicule so much and to have them at your disposal as your well-endowed private fuck object, is full of the appeal of taboo and irresistible to people in this lifestyle.