Logistics.

 It's quite common to note the appearant waste of money, however, logistically, it is likely the most efficient way to have a standardized, safety accepted method for getting the children to and from the school grounds. In the same manner as UPS or FedX, they have some deliveries that cost them money, but as long as the greater majority of deliveries make them money, they can provide a cost effective means of pickup and delivery.

 IMO.