My daughter has pointed out that I don't respond to the cashier's salutations and instead just smile and leave, but I didn't think that was a bad thing. I too thought their patter was shallow and they didn't care if I responded or not. I suppose most don't but I have had to smile back and say thank you and wish them well too or my daughter would be upset.
I had to work in a drugstore at 15 years old to pay for my own clothes and basics, etc. that my parents wouldn't supply because all their money went to alcohol. Customers at the drugstore would line up to pay, especially on the 1st of the month. This was of course in the worst part of town and they were spending their welfare checks. Lines of 25 people deep all day long. I HAD to thank them and wish them a good day but it became such a trial as many of them were drunk and unruly. The ones that wanted to chit-chat just irritated the ones waiting behind them. But then when it was their turn most of them wanted to chit-chat too. I was so grateful for the ones that just paid and left.
So I adopted that behavior of just paying, smiling and leaving thinking it was easier on the clerks. Did that for many years too. I won't go on chatting with the clerks but I do respond now. I guess I finally learned to equate it with the way people say, "how are you?" and they don't really want to know but you answer "fine thank you, and you?" and leave it at that.