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First, people who are banned from online bookies use "horses" or other not-banned players to place bets for them.

Second, the FBI is targeting real world Mafia members, who will typically be the bookies taking action from others. If they know in advance, through blackmail or collusion that an NBA player or coach will throw a game, they can exploit this versus their entire betting pool for massive wins against the suckers placing bets with them.