Because they culturally see anyone "beneath them" as NPCs that exist to serve them, and anything other than perfect servility is seen as a personal insult.

American customer service mentality is supremely fucked up. I actively avoid businesses that more actively force employees to interact with customers in certain artificial ways, because some are even worse than the norm.

Playing music is fine. Blasting music is the problem. I don't like it when anyone blasts music in their car, not just mail carriers.