It is silly but it is to decrease their scoring average. I was a master (the old version) back when St Andrews came out and I played it over and over again when we had the front nine and my average dropped from 62 to 59.
After talking to a friend on here I realized that the only way your average means anything is if you play all the courses. Today I am a Legend and have an average of just under 65. I play the weekly tournaments and some of my country club tournaments and enjoy all the courses.
I did take an afternoon a few months ago and played 18 holes on each of the four courses that we have available just to see the difference. I found that while Oakmont was the most difficult, my average was not effected much since it is a par 70. St Andrews and Kiawah are viewed as easier but being par 72, I tended to be around the same score as at Oakmont and Beth Page.