Hotels, unlike Airbnb, get to make money off of the actual room charge. They also get to charge high rates for late bookings and general "just because."

Airbnb is a different business model than hotels. The $9 service fee is something that I happily pay for their service.

Edit: it is also right there on the submit reservation page:

http://cl.ly/3r0G0w3p1G0S0c093j3d