Page 68 - Turkey Book from Meathead

And where would these juices come from? Visualize an upside down turkey. The breast is maybe 3 inches thick at most. What is directly above it? The cavity! No juices there!
If you turn your bird upside down because you want fat to baste the breasts, alas, breasts have little fat. It's in the skin, which would be below the breasts if the bird was upside down, so melting fat would just drip out, not bathe the meat.
Finally, if you cook breast down, you smush the breasts and put marks on the skin, and if you put the bird in a roasting pan, the skin will probably not brown properly. Ugly.