Cashiers have never had an issue with it. And by definition I'm middle class so I'm not
sure what point you are trying to make there, presumably that it would only be acceptable to the lower/working class?
And why would you prefer others not to eat in the supermarket? Most supermarkets at busy times have tasting plates etc for people to try and eat products in store at various counters and aisle ends. Do you also have a problem with this?