I read another article on this a couple of months ago. Another problem was that these people just blew off any court appearances, so additional court costs were added. Also, one judge they interviewed for that article said that if the person either made arrangements to pay the fine over time, or at the court appearance told the judge that it was impossible to make a lump sum payment, some deal was usually struck to either have partial payments over time or just reduced fines. Occasionally, the fine was even waived in cases of severe hardship.
These people in jail probably just ignored the tickets and then the court summons, hoping they’d just go away.
They probably owed child support... Or had other issues that would result in jail time.