This is because it is part of the criminal "enterprise". The enterprise is recognized as consistently engaging in criminal activities. Whether or not a guy gets hit with a big sentence for bookmaking is beside the point; he is still a member of, and supporter of, an organization that murders, robs, distributes narcotics, extorts, and engages in labor fraud, among other things.

An independent bookie is not going to see as harsh a sentence because he isn't part of a larger group of criminals committing other crimes.