I'm with @Liam9 , I disagree with this policy too.
It's a sledgehammer approach and not in keeping with what Airbnb branded itself as: staying in a private home with a local.
The question should be, does Airbnb ( or fill in the blank) discriminate against hosts?
My space is not equipped for children, pets or physically handicapped, my building is a walk up I do not have an elevator.
I have read some posts where people say they were discriminated against because they were declined, no mention or reason or proof. Who is going to follow up, hear the host's side? Or is this going to be another: Host wrong, Guest right, scenario, like (see posts) many a refund awarded to guest without proof for compaint. If declining / unavailability is going to be the measure of discrimination I can see more abuse and fines and penalties coming the hosts way and more negative situations for hosts who feel they cannot decline for fear of being penalized out of business.
As for instant book for all - bad, bad, bad idea. The host needs some measure of control as to who they allow into their homes. Right now on Airbnb you can be a flower, a horizon, a cat and with only an email or phone number (so easy to fake) and still be verified and instantbook! And even with awful reviews of terrible behavior you can still instantbook!
To quote @Dawn33 : "I don't care WHO you are, just HOW you treat others and their spaces"