Sports leagues believe that betting damages any sport's integrity. For this reason, leagues immediately reacted and removed any possible lucrative sports events from the state when Chris Christie revealed that the state of New Jersey was considering in legalizing sports betting.
Although sports leagues are correct in the fact that betting and cheating on sporting events jeopardizes its integrity, bettin alone is not the only culprit. When caught cheating, many players and teams get away with it instead of facing repercussions because people call it "part of the game."
A good example was when Serena Williams lost a game on purpose against her sister as she already had a grand slam title and Venus needed one for future brand sponsorships. Even though nothing was done about this, Serena continues to be one of the best tennis players. This is what is considered cheating for reasons other than gambling and sports bettors make lots of money out of something that wasn't completely honest.