I'm certainly not trolling. The buses have strong metal hooks at both the front and the rear of the ECV to secure it. It is locked into place, and cannot move while the bus is on motion. They never force a person to stay on the ECV, even if they know the bus will get completely full. I've never seen them even ask someone. They always ask them to take a seat, and the rider will state that they don't wish to transfer. A long seat belt then goes completely around the person. While it's not foolproof, since there aren't any seat belts on the regular seats, it's still more secure than the person simply sitting on the ECV with no restraint.
I know all of this because my wife, with 3 herniated disks and 2 knee replacements in her future, has to have an ECV in order to enjoy the parks at all. If she walks, she is good for about an hour, and then she is done for the day. Not just with the parks, but has to stay in bed the rest of that day. For the longest time, she refused to get one, but now realizes that we can both enjoy the parks a lot longer if she has one. She would absolutely NOT have to have one if she didn't need it - and no she isn't overweight at all. To see her on it, you would think nothing is wrong, until you see the grimaces and the limp as she transfers and has to walk to get onto an attraction.