The owners of casas, or in some cases local agencies representing multiple casa owners, set the prices. Airbnb charges the casa owners 3% of those rates and charges guests another 6-12 % on top of the rates set by the owners. Airbnb makes additional $ from holding guests payment from the time of booking until 24 hrs after check in. So at most they are making 15%, which is pretty fair. Not sure what you are upset about?
I own a vacation rental property, but prefer to use other sites that charge less and don't hold my guests' payments for months at a time.