I know I'm bringing a pretty old thread back to life, but I wanna chime in.

Michael's whole mantra was becoming "legitimate." Casinos, while considered a vice by many, were indeed legitimate, and if operated properly, were cash cows that didn't bring down unecessary heat from the police and feds. Picking up the casinos also enabled the Corleones to have more a cash stream to keep up with those families that jumped with both feet into the narcotics market, something the Corleones didn't wish to participate in.