Why do you  slice whole ducks in half when roasting but not for other poultry?
I agree with Jay's answer that one of the reasons is because of keeping the skin crispy, but I don't agree about the difference with other types of poultry and have a bit more background info.

The root difference between duck and other poultry is that duck is much fattier, and most of that fat is stored under the skin. If you don't do anything about the fat, you'll get a bird that will be too greasy to enjoy; and also the skin won't crisp as nicely as, say, chicken skin would with similar preparation, because the skin will be soggy with the fat that's rendering underneath. There are more ways than one to get rid of the fat; a popular one is to steam the duck before you roast it, usually after sticking a fork into the skin a few times so that the rendered fat can get out easily. This seems to be a different solution to the same problem: if you cut the duck in half and put the cut side down (which is the same as putting the skin side up), the fat will have a much easier way out than if you cooked the duck whole.