I think arresting people for cheating legitimizes backroom / mafia gambling. All the other rings (and those left from this one) can say "Look, those other guys got arrested. The law protects you. We don't want that to happen to us. Our game is definitely fair." Of course, they too are cheating.
The only reason the FBI cares here is probably because one of the victims had pull. If you or I get cheated, the FBI won't care about that.