They should say, it's bad manners and this is their job. The other one that rarely happens but is rude is when 2 cashier s carry on chatting to each other whilst serving you. Doesnt really annoy me, but I just note it as rude behaviour, poor manners. I'm British, so perhaps more attuned to this kind of thing.
Slight tangent, but reminds me of something that happened in the super market 2 weeks ago. I was waiting at the end of the checkout for the attractive check out girl who was probably early 20's to scan the customers food, she asked me to put the dividing thing up, I just said 'No' with no humour in my tone/face, she did a double take, looking shocked and said 'excuse me?'. I just said No, this time with a grin. Then put it up for her.
When she served me, she said I can't believe you said that to me… no ones ever said that before. I just laughed and said I thought I better do it before you leaped over and start wrestling with me. She started laughing and it went well from there. I didn't have my grocery card to put points on at the end, she said when I find it to come back and see her and she'll sort it out for me. COuld mean nothing, but thought it interesting why she said come and see her and she'll sort it, rather than head to customer services or just back to the shop. Definitely sensed interest/intrigue from her.
Interesting how her demeanour changed and how well the interaction went after. Ties in with what some mentioned above as saying something different/odd to make positive impact/impression with them.