When someone deals day in and day out with abusive customers who are sure of things that are just flat wrong it has a predictable effect on the persons handling of legitimate cases.
It's not good, it's not just, but it's understandable. Armed with that understanding we can at least try to self-help by making ourselves look as little like the bad customers as we can. Should we have to? Not according to how things should be. But how things should be isn't what counts in the world, how things are is what matters. And at the end of the day-- do you want your package or not?
I think I'd also say that abusive from the perspective of the recipient isn't a bright line and you'd be hard pressed to find a culture in the developed world where the misguided interactions don't begin to feel somewhat abusive to the recipient after dealing with them day after day.
Of course, there is also outright abuse that is tolerated in the US and probably ought not be which only makes the matters worse.