So the reason you don't do this is because cart returns create a cluster of obstacles. Many people just go cart curling, aiming generally for the return and walk off, which can make an insurmountable obstacle for the handicaped person. Also handicapped spots are at the entrance to the store - if someone is going to return their cart, they'll go the extra 50' to do it. If they're a fuckin asshole that doesnt return their carts, its better not to give them a target around which to cluster their assholeness.