I think the officer playing gambling could order other people to place bets while he remained in place watching if there was news of an ambush so he could immediately tell his friends in the casino to stay away for a while. The officials were very careful if they wanted to place a bet and would wait for the right time to come to the casino because they knew their fellow officers supervised all forms of gambling. That's why it's not easy to find where cockfighting is because only certain people are invited there.
This is the advantage of being a police because they know if there will be a raid that will happen and they can leave on that gambling place before their co-police came in the scene. It is still risky because what if someone reports them to their head? They can be fired out in an instant. That serves as a punishment because they are supposed to stop these kinds of activities, not to join them and encourage other people to do the same thing. This destroys their image as a police.
I really don't know the reason on why cock fighting is illegal while other gambling games are still illegal. I think this is because there's an animal that is involved or harmed here.