For the same reason you put a gift upside down before you wrap it.To provide a neat appearance, a hamburger is placed upside-down on its wrapper, then the ends of the wrapper are brought together on the bottom side of the hamburger. The hamburger is placed ride side up in the transfer bin for sale.
If the burger was placed on the wrapper upright, the "loose ends" of the wrapping paper would land atop the finished product, giving it an unkempt appearance and threatening the unraveling of the paper. Using the preferred method, the loose ends of the wrapping paper end up on the bottom of the wrapped burger as it is put in the bin for sale, allowing gravity and the weight of the burger to hold the loose ends in place.
http://en.wikipedia.org/wiki/History_of_McDonald's
