You are probably thinkiing from a point of view of: I am in my motorized wheelchair all the time and am a pro at it. I am a careful person who will take care not to hit people.
Disney is thinking: Grandma is using a
ECV for the first time and doesn't have control. She doesn't have a good idea where the front and back are. Others often jump in front of
ECVs not understanding they won't stop.
So Disney thinks it best that there are no motorized vehicles in the area because they can't know who is "good at it" and who isn't. For everyone's safety they don't allow any. However, they do make sure you can ride the attraction. You were offered a manual wheel chair and provided someone to push it.
Does that take away your independence? yes But does it allow you to ride the ride? also yes.