Thanks, Wayne.
Richie obviously was a civilian, and he was scared of what was happening ("Carmine--NOOOOO!"). The Rosatos knew Richie would be questioned by the cops. He'd be terrified to rat out the Rosatos, so they gave him something to feed to the cops: "I didn't know the guys who did it, but one of them said, 'Michael Corleone says hello.'" That'd set the cops after Michael; and even though he had an alibi, the investigation and possible publicity would be another crack in his "legitimate" facade.