Comment by qingcharles

All the people I was locked up with for "being poor" who were serving a sentence of conviction (as opposed to those in pretrial detention for inability to pay bond) were there because of child support payments.

Basically what happens is that often the court makes a poor determination of how much a person can afford to pay each month, and when they can't make the payments they are jailed for contempt of court (for violating a court order). Often the first thing states do is take away the person's driver's license as a warning before jailing them. This makes them unable to get to/from work and usually results in their unemployment.