The only question is why the gambinos didn't kill him now that he is on the street.
Of course the mob today dont whack the rats but damn he was the underboss of gambino family and what problem could have Calì and the sicilians if someone will shoot Sammy in the head or worst?
The feds were already throwing enough money at the mob in the US to cut their nuts off.
A high profile hit on gravano would have been their wet dream. Guys would have made careers off of something like that. This is America, there are limits.
