When it was introduced to MTF, probably but not when it was originally introduced to WTAWorld now known as tennisforum
this is the introduction
PICK-A-WINNER (PAW) - INTRODUCTION
Welcome to Pick-A-Winner (PAW), the Predictions game!
The rules are simple: Just pick the winners of any 13 Main Draw matches. You choose the matches and you pick the winners. You get more points for picking low-ranked winners than for picking high-ranked winners.
then in 2003 people who have picked the game on tennisforum from Simon have decided
to change the original rules
and in the end of 2003 the new rules (as we know them now) have been introduced
and were in use from 2004.
I never found info why have they decided to come up with that stipulation. On the previous page I have written a post about that.