It has been said that Sumo, the Japanese fighting sport is not as clean as it should be. Apparently, in the maximum cathegory - short of a premier league - there is tendency to let win those who need to win in order to remain in that cathegory. This was studied and mentioned later on a book called Freakconomics.
An article here-
The references in the article you provided are dated over a decade ago, the things can be different now. But nonetheless, why there wouldn't be rigging in sumo? If other combat sports are so easily rigged, why can't this one be?An article here-
Do you feel that knowing that a sport may be rigged you could actually turn it in your favour?
Obviously, no. Unless someone from the inside spills the beans. In another case, rigging will more likely be a disadvantage, because you place bets basing on what you know about the sportsmen, the teams, their previous fighting, etc., and if the game's rigged the things become unpredictable. 
