Is there a SPECIFIC law about making fun of retarded people?

When me and my friends get drunk we go to this convenience store where they have a retard working. We always call it names and steal it's broom but when we are leaving we buy it a treat and it doesn't seem to mind. My friend said the store owner called the cops on him tonight when he went in there. All he did was throw candy bars at it and the store owner said there was a law against picking on retards. True?