Most real lineups consist entirely of criminals too. One person in the lineup is the suspect, and the rest are inmates from the local jail. You implicitly ide

Most real lineups consist entirely of criminals too. One person in the lineup is the suspect, and the rest are inmates from the local jail. You implicitly identified the reason for this: everyone in the lineup needs to look equally sketchy, or else picking out the suspect is too easy and weakens the evidential value of a correct identification. It's also just the easiest way to find volunteers to participate.