Ye Olde Gaol

Why did only the poor get sent to debtors prison? Classically, debt is class game. Look at Newt Gingrich for example. He blows out Tiffany accounts, he plays fast and loose with the campaign finance law. He bounces checks left and right, but he's a player. Like in Vegas, back in the day. You wouldn't get your knees whacked as long as the house thought there was a chance you could pay them back.
It was like that in Dickensian England. As long as you kept up appearances and played the game, the worse thing that would happen to you was perhaps you would be shunned at your club, but, if you were truly poor and fell into debt, you could be banished into the nether world of the Debtors Prison. Entire communities sprang up in the prisons. Entire families were imprisoned because there was no welfare system. Women and children were doomed on their own. It was a deadly cycle of working in the prison to pay the prison system. The jailers were paid a fee. When the Fleet Prison was finally closed in 1842, some of the inmates were found to have been in for over 45 years.
Ancient Greece closed it's debtors prisons around 600 BC but there were official Federal debtors prisons in America, but they were closed in 1833.  Sometimes an imprisonment could result from 60 cents of debt.
The states on the other hand had debtors prisons into the 1850's Six states (Arkansas, Arizona, Illinois, Indiana, Minnesota, and Washington) allow debt collectors to seek arrest warrants for debtors in default if all other collection methods have failed. Whether a debtor will actually be prosecuted or not varies from state to state, county to county, and town to town. The individual is taken into custody and is typically required to submit financial documentation to the courts (to facilitate seizure of assets or wage garnishment), although in some cases the individual may be held indefinitely until a payment plan is reached or the debt is paid in full, especially if the individual is insolvent. Other states have outlawed this type of collection action (Tennessee and Oklahoma have ruled it unconstitutional)unless the court finds that the debtor actually possesses the means to pay—except in the case of child support obligations
Most state constitutions  including Minnesota's, have clauses dating to the 1850s that expressly prohibit the jailing of people for their debts.  Some people make the claim that it is unconstitutional in the United States to incarcerate someone solely for failing to pay a debt. However, there is little settled law on this matter and plenty of precedent for de facto debtors' prisons.

More than a third of U.S. states allow borrowers to be jailed for non payment of debts. Judges have signed off on more than 5,000 such warrants since the start of 2010 in nine counties.Because of “sloppy, incomplete or even false documentation,” many borrowers facing jail time don’t even know they’re being sued by creditors. 

I read this particular story on The CBS web site. Briefly:

How did breast cancer survivor Lisa Lindsay end up behind bars? She didn't pay a medical bill -- one the Herrin, Ill., teaching assistant was told she didn't owe. "She got a $280 medical bill in error and was told she didn't have to pay it," The Associated Press reports. "But the bill was turned over to a collection agency, and eventually state troopers showed up at her home and took her to jail in handcuffs."

17th Century English Debtors Prison by William Hogarth

Although the U.S. abolished debtors' prisons in the 1830s, more than a third of U.S. states allow the police to haul people in who don't pay all manner of debts, from bills for health care services to credit card and auto loans. In parts of Illinois, debt collectors commonly use publicly funded courts, sheriff's deputies, and country jails to pressure people who owe even small amounts to pay up, according to the AP.

Under the law, debtors aren't arrested for nonpayment, but rather for failing to respond to court hearings, pay legal fines, or otherwise showing "contempt of court" in connection with a creditor lawsuit. That loophole has lawmakers in the Illinois House of Representatives concerned enough to pass a bill in March that would make it illegal to send residents of the state to jail if they can't pay a debt. The measure awaits action in the senate.

Illinois isn't the only state locking up residents for being too poor to pay their bills. A report from the ACLU found that Georgia, Louisiana, Michigan, Ohio, and Washington were also doing this, and at "increasingly alarming rates."

A report from the New York University's Brennan Center for Justice found that states are also adding "poverty penalties," including late fees, payment plan fees, and interest:

Alabama charges a 30 percent collection fee, for instance, while Florida allows private debt collectors to add a 40 percent surcharge on the original debt. Some Florida counties also use so-called collection courts, where debtors can be jailed but have no right to a public defender. Being denied a public defender would seem to be a blatant constitutional violation. If you are of the 1%, you are rewarded for debt. You are encouraged to incur more debt The only people being prosecuted like this are people too poor to pay their bills.