A manager can influence a baseball game in many ways. One key way is in deciding when to play and when to rest certain players. It's not difficult to conjecture that a manager with a bet on a game might be more apt to not rest any of his key players that day. And he might be more apt to use his closer, maybe even bring him in in the 8th instead of the 9th. Maybe use him for the 3rd day in a row when normally he would not. On the other hand if a manager didn't have a bet on a game he might be more apt to pick that day to pull some of his regulars, not use his closer, etc. The existence of the bet is just too likely to exert some influence on the manager and that effects the integrity of the game. Even if the manager only bets on his own team. MLB is wise to have their anti-gambling rule and to have the lifetime ban as the penalty. It should be one strike and you're out ont his one.