The reason: grocery shoppers are likely to be toting heavy frozen turkeys, which could easily be used as weapons.

This lesson comes to you from Raleigh, North Carolina, where "authorities say a shopper clubbed an alleged carjacker with a frozen turkey as he tried to steal a woman's car in a grocery store parking lot Sunday."  Thirty-year-old Fred Louis Ervin was savagely beaten with a bird wielded by a holiday shopper who came to the aid of Irene Bailey.  Ervin got away with Bailey's car, but was captured a short time later.

Ervin reportedly suffered "serious head injuries" from the turkey-bashing, but was said to be resting in good condition at a local hospital.  The hero shopper was not identified, nor is it clear whether the turkey was still in edible condition, although that seems likely if it was actually frozen or semi-frozen. 

In fact, I would bet that a Thanksgiving turkey tenderized on the skull of a carjacker would be very, very tasty indeed.

Link: AP via FindLaw.com