This is due to the problem with volume forecasting in part of bookings that are not kept. Agencies oversell their inventory because they know many bookings will never be used. This is especially common in cities with large volumes of tourists who shop around for best prices.
I firmly believe all rentals should have cancellation fees just like airlines.
Just so I understand, if it means someone doesn't have to wait in line, they should double book which will reduce inventory and generate higher rates for others? Seems selfish and inconsiderate to me.