I guess the answer is "it's easy" and "it makes ave. reduction easier. It's at least easier than to follow the guidelines for 9 hole comps in real golf. There, in Germany, the "unplayed" 9 holes are counted as Par (18 net Stableford points).
Imagine the fuss involved to add the respective strokes for Par for the unplayed holes, ranging from 34 (OLY front nine) to 36 (KIA, STA and others). (Where are 35 correct?)
Furtheron, as you said, 30 are easier than 60 which adds to the satisfaction of the customers and thus to WGT's business success.