I also think DAs use this language because it comports a certain exotic criminal element to the defendant that sounds better than calling him "the boss." Remember that it was a jury trial, so the first thing the defense attorney did during jury selection was ask every juror if they had ever heard of Joey Merlino, knew anything about the Mafia, or had any interest in mob lore. So did the jury see the old-school mafia chart about where Skinny stood in the family? No chance. So the prosecutor said he was involved in organized crime and used a term ("capo") that sounds a lot more dangerous to your average civilian than "boss." After all, your boss is not involved in organized crime, but a "capo" might be.