ken said:
^^ you pay by cash and no one can monitor. YOu pay electronically they know what you bought, where you bought it, how much it cost. At best, supermarkets could get access to this information and give them all the marketing material they could wish for and then predict you down to a mathematical equation.
governements could use this information to dirty up the names of political opponents.
As long as they don't go buying pornography or drugs from their local supermarket, I don't think it'll be too much of a problem.