[whoopdedo]

> One does not thank an ATM...

Ever use an ATM with voice prompts? It's hard not to talk back to a machine that's talking to you.

It seems to me that the service-oriented "thank you" is more about expressing satisfaction than gratitude. It's a way of communicating that you have no complaints about the service without suggesting that there could have been anything to complain about. When a person doesn't say "thank you" it implies they were unhappy with the transaction and that they may be less likely to want to do business with you again.