Why does the pin recess on the Technic turntable continue to the edge of the part?
As suggested by bumbumfish, it is indeed required by the molding process to be able to remove the part from the mold. This does exist in many other parts, such as Technic frames:



The alternative is a mold with moving parts, but this is more expensive, and can't be used anyway where there is not enough room for the mechanism (e.g. inside frames).