My guess is that many of their customers want to run Oracle software inside VMs. Probably big server farms that want VM containers it can deploy, boot, and reboot quickly as needed. For enough VMs, a VMWare or Parallels license will add up.
First, I think many engineers at Oracle use it to test software locally without having to fallback on complex solutions. Second, it's probably a small development team, maybe three or four FTE, and how much money is that for a company like Oracle? VMware licenses are probably more expensive.