I'm not so sure that's a good argument. Are you saying AirBnB is a business that helps people break their leases and condo regulations to make money, and that we should keep this covered up?
Plus in this case, there was no discount: the low-paying customer was ditched as soon as the renter found a higher-paying one. In times of scarcity (big event in city), a leveraging tool such as AirBnB will be used to charge more, not less (AirBnB allows the renter to find more customers than the low-paying ones, right up until the last minute).