I don't think standing all day like that is necessarily better, as it can cause other health complications. I did a light google search (i know, not the most extensive research) and the results suggest that standing for long periods can cause joint pain and put folks at a higher risk of problems with vericose veins. I understand that cashiers may do other things around the store, but some stand at the register for hours on end and that's not much better than sitting.

https://www.webmd.com/fitness-exercise/standing-desks-help-beat-inactivity