Typically all these long packages are damaged on conveyor belts at the various sorting facilities and distribution hubs. They spin sideways and get hung up on the conveyor supports, and the weight of other packages behind it is what breaks something.
It will even fold up a thin wall PVC pipe on occasion, but the PVC is much stronger than cardboard..... But it's real important using corners taped to that pipe so it doesn't roll around easily..... So its not really a USPS vs UPS deal.... it's the conveyor belts that are doing it.