"But in jail (jail is a term for institutions that hold people no longer than one year, in most cases, a majority of a jails population is transient), how do they know who the murderers and wifebeaters are? Isn't that the point of our justice system? Everyone is treated innocent until proven guilty?"

I explained it too shortly in my comment in the interest of brevity, sorry.

The jail I visited was also a Prison, it served both purposes. The bottom floor was a holding center (jail) while the upper floors were a prison.

In this case, when a prison inmate needs to be transferred to another prison -- say they've completed 5 years in county and need to do 10 years in federal -- they are moved downstairs to the holding facility (the jail) that is shared with say people being picked up off the street to hold.

In this case, they stored me, a young naive programmer who forgot to pay a ticket, with hardened convicted cop killing murderers who had completed their time in the prison and were being transferred to a new prison.

This was a county prison for a top 10 metro and a major city which has gang violence, so these were convicted and often gang related criminals.