It's less about smugness and more that customer-service people are expected to behave like AI and always have an answer whether it's correct or not. I hope at some point we can drop the facade and "I don't know man, I just run the till. Do you want coffee or not?" becomes an acceptable response.
I can't speak for the US but people absolutely do not act like this in Europe (and Europe has some of the most arrogant self-assured service workers on the planet depending on the country - I won't name names but you can probably guess which country has the most extreme and limiting arrogance around what should or should not be done with coffee).