They need to do it so that you can only challege someone who is the same rank as you (i.e. begininer, or Grand Master). I was challenged by someone in Meriball, and accepted because I thought it would be fun. The person has a Meriball trophy, and my best score is as follows
Meriball 260 Points Novice (advance at 300)
I believe his highest was 900-something. I know there is something they could do to atempt the prevention of people taking advantage of this.