Sam still can see the angle: He has deep knowledge of the underworld, He understand how gambling world works. For this understanding he still can earn lot of money and most of all he can send money back home to Bosses. Well, no bosses will will kill a golden goose.
Besides Sam never betrayed the bosses. You see even when Nicky tried to persuade him to take over but Sam refused to be part of that.
Another thing that Sam directly didn't help to skim the casino. He just made absolutely sure that the casino earns money. So I think Sam couldn't be a witness and he is not considered as a threat.