Bumper problems of the 1999 GMC Suburban

The vehicle was parked on a slight grade in the driveway. When it was shifted from park to drive, it drifted backwards approximately 2' (1mph - maybe) where it contacted a sapling tree with the right end of the bumper. At first glance, no damage was detectable. On closer examination, it was discovered that the end of the bumper had bent inwards towards the vehicle and just narrowly missed contacting the rear quarter panel. Further inspection uncovered that the top of the bumper had buckled under the step cover near the center of the bumper. When the dealer was contacted, they recited the gm policy statement that the bumper only withstands impacts that are squarely applied at the very center of the bumper. Customer service at GMC waited until contacting the dealer to recite the same finding. Most people purchase this truck for the size and safety of a larger vehicle, and certainly one would expect that an SUV would be sturdily built. I want the manufacturer to replace this bumper under the vehicle warranty. This is undoubtedly a safety hazard as a 5mph impact would have had a devastating affect on the vehicle and its occupants.