I don't look like I could use a "cart" and I DON"T use one either. I have very bad arthritis in my knees, bone on bone with no cartilege left and very young to have had this happen. When I go to Disney I take my arthritis medications of course, a pain medication and wear pain patches on my knees UNDER long pants. My DH sometimes begs me to use a cart, and it's because of people who give dirty looks or "think" mean things, that I refuse. So I LOVE DisneyWorld and would rather walk around in pain, sitting down frequently to take the edge off, than use a cart and have people thinking I don't need one.
So perhaps, for every person riding in those cars that don't need it, how many are like me, walking around in pain because of what people might think and how they might look at them. I don't know what happened in the incident you described, but I do know that most people that need the carts, probably don't look like they do.
And walking to the gate is one thing, being on your feet for even one hour gets painful and by the time I've been in the parks for 4 or 5, I'm sometimes almost in tears when I get back to the room. I guess in this case it might be literally better to walk in someone's shoes for a while before judging.
So I don't use a cart because of "comments" and things people might think, but still go, because I absolutely LOVE Disney and will not deny myself that, even if it hurts.