I always take the a neutral side. I do the best I can to provide the player with the necessary channels and resources to pass their complaint however, until I hear the side of the casino, I stay neutral.
With these cases you have to see how things develop. The police will always do the same asking both sides for their stories and then confronting them and deciding what to do.
To defend the people making these accusations, I saw a few situations where it was impossible to tell who's right. The player claimed he was banned for nothing, the casino said it was because they found ways of him trying to cheat but wouldn't provide any proof because it would help other people cheat. How would you decide who is right?
In such situation I usually take the side of the client. The casino should provide proof and explain why the player was banned, especially that they confiscated his money.