Not the hosts race, the potential renters race. In order for Airbnb to say someone of race {race} got rejected, they have to know, or guess, their race. Whether they ask for that on the application or look at the pictures, its probably illegal.
They could add button "Report discrimination" for rejected potential renters and then ask them what type of discrimination they want to report. Then they wouldn't need to guess anyone's race.
Exactly. There is no serious reason why they can't do this.