It has nothing to do with safety. It is usually either steel or polyurethane. But japanese cars are way safer because of the engineering of the car. Just because a piece of metal is thinner or bends more easily in a parking lot crash, it doesn't have any bearing on the actual ability of the crash structure to handle loads. All car manufacturers do lots and lots of FEA- Finite Element Analysis so they can engineer the car's safety as a whole rather than by component. As a result, cars are safer now than they've ever been.It's simple physics, deforming a structure absorbs energy which could otherwise be transmitted to the occupants. If you had an indestructible car, it would be LESS safe in a 30 mph crash because all the decelerative loads get transmitted directly to the passengers. Stopping in 2 feet is much better than stopping in 0 feet.