I don't know if this is the single source, but in his book, Leonetti claims Joey was welching on bets with connected bookmakers and lied about taking the bets when confronted. I also heard Anastasia say something like "Bookmakers would dread seeing Joey come around", so I think there's basis to believe that, but we don't know for sure I guess.

As for the topic generally, I side with those who asserted that this isn't a good business decision... It simply wouldn't pay to fleece someone(s) when the business by itself is a big money-making machine whose main pillar is trust.