--First, the Mob doesn't go after legitimate cops--it's too dangerous. Recall that Charlie Luciano had Dutch Schultz whacked because Schultz was going to kill special prosecutor Thomas Dewey--even though Dewey sent Luciano away for 30 years.
What about the Mob looking guy on the stand who gave Pistone the gun on the head hand sign in the courtroom while he was giving his testimony during the trial?
Also, the show said Pistone now has a new identity and is living in an undisclosed location. So I suppose he's still a wanted man by the Mob.