I have seen them only use one tie down if the person isn't staying in the ECV, it happened several times with me.
I have seen them use all three every time too. It really is a safely thing too with just one tie down much easer for it to move flip or do something. If I every saw them just use one or two I would say something quite loudly then get off the bus.I have never seen that - seriously. I have stayed on my personal device maybe ? twice in all the times I have ridden a Disney bus, and Every. Single. Time I don't sit on the device, the driver has used all three ties downs to secure my (now riderless) device. Every. Single. Time.
If you are experiencing anything different, I would get the drivers name and bus number and contact the Transportation department ASAP. That driver needs to be retrained.