Professional athletes and sports players weren't allowed to bet on them or upon the game they play. This is done to have them self control, because once they start placing bets on them, it'll be the beginning to play for his/her favour. Finally his game game will be much based on personal financial gain which is not good while representing a team or a country. For this reason professional players weren't allowed to make bets, but these days there are chances of professionals making bets using the anonymous betting opportunities.
I think you mean the other way around. Professional athletes, if allowed to bet on themselves, would likely throw the match and let the opposing team take the win for easy profits. This is way easier to do and less effort involved too because they just have to play bad in order to get some payday for their pockets. Professional athletes sans betting is highly motivated by their desire to be at the top of the sport, with prize money coming in as the second motivator. If money is easy to get on professional sports, do you think athletes who like to throw games will still exist? I think not.