I would guess Disney has a contract they make you sign when you rent a scooter that has some pretty strict language regarding YOUR responsibility for whatever you run into.
And as for training? “Here’s how you make it go and stop. This is how you turn it”
How much “training” do they have to give you? A multi-hour course with a driver test like the DMV?
Im going to guess that the victim didnt sign a contract limiting Disneys liability if someone driving a Disney-owned-and-maintained scooter should run him down.