This is what an aisle should look like from the rear of a commercial airliner. In the event of an emergency landing, like Captain Sulley's famous Hudson River landing, the aisle is clear for exiting. The flight attendants pace endlessly to insure no bags are obstructing the path. However when an obscenely obese person boards the flight, all those precautions can go out the window.
As this Mark Mangino clone demonstrates, an obese passenger creates a hazard for all on board. Attendants can't get by on foot, forget bringing the food cart down the aisle. The unlucky fellow beside him is in hell and in the event of an emergency no one has a chance. Forcing the passenger to pay double is the obvious option, but implementing it is a nightmare. Legal and logistical obstacles abound. How fat is too fat? How do you enforce the weight limit? Most people check in online or via kiosks, therefore airline employees don't often see the passenger until boarding. Can the purchase of the tickets be conditional on verification of proper size? The solution is elusive but the problem is real; unless the super fat decide to buck up and buy two tickets the airlines need to address this danger.