imo that doesn't make sense. All the online betting platforms will cut off the sharps. If you are net profitable and you make too much money from them, you will get banned.
There is also the very strong possibility that they are colluding with the online betting platforms in some way. Coupled with the fact that any difference-maker athlete is getting a huge salary, and blackmail/extortion becomes your best option to getting one on your side.
However, if you assume they were feeding the information to the platforms...
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.