It's a similar model to many taxi cabs. The driver is the owner, purchases (or leases) the vehicle, and purchases the medallion. Then he pays a (significant) cut to be a part of a "dispatching co-op" so he finds work.

With SuperShuttle, the driver is generally the owner. He buys the van then also pays SuperShuttle a cut (which I believe is both fixed fee + commission) to be the dispatcher. I tip the SuperShuttle driver in a similar way to a taxi driver, as the economic model is similar.