Prisons serve multiple purposes. They are not merely there to hold scary people. Indeed, most scary people are not in prison nor should they be. Being scary is not a crime.

Prison can be an entirely appropriate punishment for nonviolent crime. For certain classes of crime, it seems the most appropriate punishment because it can be difficult to punish effectively with fines. If you only fine for financial crime, you make lucrative financial crimes more appealing. Worst case they just take your ill gotten gains.