> aren't really expected to talk

This is an assumption that I would interpret as rude if I were the cashier

Agree, it definitely is rude. Saying hello, good afternoon, responding to a question about cash/card also involves communication, and with earbuds it is simply rude to the other person.

If you don't want to talk to anyone, go to self-checkout or a vending machine, but cashiers are humans and not just robots scanning items.