I've posted this before, so please bear with me:
I also read the story about how Danny Aiello ad-libbed "Michael Corleone says hello." Why did an ultra-careful, fanatically attentive director like Coppola leave the line in (to the eternal bafflement of people like us)? Perhaps because the line was intended not for Frankie's ears (he would be dead in seconds), but for the ears of Richie the bartender, whose place was being used for the murder. Richie is a "civilian," and he's nervous as hell ("Carmine-- NO!," he screams as Carmine draws his gun when the cop walks in). The Rosatos knew he might be squeezed by the cops during an investigation. He wouldn't dare rat out the Rosatos, so they left Richie with a line he could repeat to the cops: "The killer said, 'Michael Corleone says hello.' " That would set the cops to questioning Michael. It'd also be reported in the press, which would further damage Michael's claim to legitimacy.