One problem (of many) is that there are so many laws that everyone probably breaks a few every day. Did you cross the street to talk to your neighbor? Jaywalking. $50 fine. So, a system of laws has been created that allows local governments to almost arbitrarily tax their citizens at will simply by choosing which laws to enforce.

I believe there are restrictions on the work that prisoners can be forced to do because there's a fine line between incarceration as punishment and enslavement. If the government has an incentive to incarcerate, it will seek to find ways to acquire more prisoners. I see this as a parallel to fine collection.

All of my comments in this thread are US-centric, obviously.