I always thought it was because you couldn't kill a cop. But for some reason, a retired or ex cop was a different story.

That's why Tony waited until right after the retirement party to have him whacked. Imo, there's no doubt that he was Dickie's killer. As others have said, there was no reason to lie to Chris as he didn't need a reason to kill upon command.

You shit-kicking, stinky, horse-manure-smelling motherfucker you! If you ever get out of line over there again, I'll smash your fucking head so hard you won't be able to get that cowboy hat on. You hear me? Fucking hick. -Nicky (Casino)