I think it's a myth.
Honestly, now -- have you ever really seen anyone walking down the street with their arms stacked high with wrapped Christmas gifts?
It's a nice image, but if it ever did happen, it doesn't happen anymore. Today's shoppers lug around plastic bags stuffed with gifts that need to be wrapped after they get home.
Some stores hand out free gift boxes, but those will still have to be wrapped by the customer.
I do remember when many of the stores wrapped gifts for their customers. They had big rolls of wrapping paper hung on the end of the counter, and they wrapped them on the spot, right after you bought them.
I'm not sure why, but it seemed that the stores usually had striped wrapping paper at Christmas.
One time, a lady clerk showed me how to make fancy, spiky bows, and I still make them sometimes when I want to make a package look really special.
I must admit that I'm glad clerks don't have the responsibility of wrapping the customers' Christmas gifts anymore. It would definitely require a lot of time and a lot more counter space than my workplace offers!