PS. As I am writing this, another possible explanation comes to my mind - the volume of stuff being purchased. It is much higher in the US than elsewhere, for several reasons, chief of which is the land use. Europeans and South Americans tend to walk to the stores every day and buy only the stuff they need that day. US-sers tend o drive there from the bowels of the suburbs once or twice a week and hoard stuff for longer time periods. The volume of stuff that need to be moved throughout the scanners and into the bellies to the suburban SUVs may indeed requite the extra flexibility that you mention.
I like that explanation - it is materialistic and does not rely on culture and psychological preferences.
Wojtek
----------------------------- Except that your alternative explanation doesn't conform to observation. Checkers stand in every mom and pop, neighborhood grocery store that I've ever been in. They also stand in convenience stores and liquor stores, where people just pop in a buy a couple of items as needed. I think it is a cultural issue, that may be traced back a long way: American business owners just don't like the idea of such ostensibly menial employees sitting down, a deep-seated notion that "I'm - or we're - not paying you to sit on your ass"