There's nothing imaginary about it. Discrimination in housing has been illegal since the 1970s.

Airbnb has found (accidentally) an unforeseen loophole in the law, where a practice that had only a marginal impact on the distribution of housing is on track to make a more significant impact. They know as well as anyone else that the law is going to catch up, unless they keep human nature from using Airbnb as a vector to re-segregate a big chunk of the housing market.