I would guess that probably since day one WDW has been using charter buses owned and operated by Mears. Probably not much in the beginning because bus usage at the time was fairly limited and the equipment was all new. But when the Resorts went up and the free ride from the airport started, they have been using charter service for that and the Cruise connection. They use the charter for onsite tours (sometimes the cruise buses, since they might be not busy on that day), but, they are as much a part of Disney Transportation as the ones with Disney painted on the side and are strictly onsite transport buses.