Tell that to the thousands of people locked up for inability to pay their child support. They don't imprison you for the debt, they imprison you because the court gave you a direct order and you could not fulfill that order, thus you committed contempt.
Sadly it's not that simple, although that perhaps is the way it should be because that would be a form of tax evasion.
All the people I know locked up for child support issues just couldn't work enough to afford to live and pay what the government was asking, and the government would fight to keep the level the same, even though it wasn't sane. Then the government would first punish these people by canceling their driver's licenses as an incentive to make them pay, which would often result in them losing their jobs and falling further behind. In the end, jail.
