I often think this, but to be honest I am a hypocrite as I worked in Asda and Greggs where I was fully miserable. Miserable because of the enviroment and tired of repeating myself to people who, the opposite way round, ignored me or complained or were down right arrogant because they thought they had the power to do so!
But you are right, it spoils it for other people who are pleasent, if shop assistants get a line of rude people they naturally become defensive or have a couldn`t care less attitude as long as they serve people.
I personally hate consumerism in all its forms, I think people are bullied in the work place because of the pressure behind the scenes and you have to have a thick skin to work there.
I decided I wasn`t a nice person in retail work, in fact at times I was pretty nasty and it was effecting my home life too, so I decided to get out of it.
I agree it doesnt hurt to have manners though. There is a girl in the fruit and veg shop that I go into who just grunts lol. So I think if you REALLY don`t want to communicate with people you should work elsewhere