The transaxle followed by the engine and those pieces are connected to the main frame at 3 points. My guess is that the bumpers are so flimsy that an hit at over 5 mph would crush them like a soda can. The bumper is great protection from a accidental hit at very low speeds, like when parking, or a very light tap from behind, but IHMO does nothing for hits that would likely cause bodily injury.
Black Ice -
the bumper assembly is very substantial and is attached directly to the end of the frame behind the transaxle.
It is a work of art when seen w/o the covering.