The homicide rate is a clean way to compare the criminality of different populations, because it's easy to define and hard to hide. That way you don't get lost in the details of what's a crime in one country and isn't in another.
Given that America has a homicide rate an order of magnitude higher than its peer countries, it stands to reason that its overall criminality is higher, and therefore we can expect to see more people imprisoned per capita than in those other countries.
Given that America has a homicide rate an order of magnitude higher than its peer countries, it stands to reason that its overall criminality is higher, and therefore we can expect to see more people imprisoned per capita than in those other countries.