Vito Corleone and his world had no use for the judicial system.
You're right, they wouldn't sue McCluskey for acting as Sollozzo's body guard. But they did use the judicial system. Why else should Vito keep all those judges "in his pahket"? And he insisted that Tom became a lawyer. Also, I believe, they would have used the judicial system if McCluskey interfered in the renewed protection of Vito at the hospital. This wasn't just an empty threat.
