SinkingFast said:
Also, what's up with just forgetting about the money they owe? Why not put the debt on hold until they are released and can go to work??

Does the time spent in jail pay for the additional crime(s)/new charge(s) as well as the former?? Please explain how that makes any sense.
As a practical matter, if you put someone in jail and then tell them they owe a debt when they get out, what are you going to do if they don't pay it? Put them back in jail again? And how often are you going to repeat that process? Even private companies eventually write off bad debts.
At some point you just have to realize they don't have the money, and they're not going to come up with it. You also have to consider that every time you send the police after them, and pay to keep them in a cell, you're using resources (police and cells) that could be used to protect the public from violent criminals, rather than people who can't pay their debts.
As a technical matter, when you revoke someone's probation, that probation is over. The prison sentence is in lieu of the probation. (In other words, they're getting prison instead of probation.) Prison is a punishment for not doing what they were supposed to do on probation.
Many times the judge can, if he chooses, order them to pay a fine, in addition to their jail time. However, that gets back to first problem - if they can't pay when they're on probation, how the heck are they going to pay when they're in jail? In any case, a judge can only order what's authorized by statute. Usually that's jail time and/or a fine.
If a judge does order a defendant to pay a fine in addition to jail time, what often happens is either the defendant stays in jail longer (because he can't pay the fine) or his family pays it instead. Neither of those is really a good result, since neither punishing someone's family, nor keeping someone in jail for being poor, is really the point of the criminal justice system.