I think rusty was about sending a message because it happened after carmine and johnny had reached a truce. They reached a truce at the end of season 5 and rusty got whacked after johnny's daughter's wedding in season 6.

It was johnny sending a message that he knew rusty was behind carmin'e move and he wasn't going to stand for that kind of mischief in his family. Also because rusty made a move once through carmine he had proved to be a threat to the thrown as long as he was alive.