I think the common complaint here is that there are plenty of "crimes" that are committed on a daily basis like minor speeding (going 1-5 mph over the speed limit), jaywalking, etc, that are nearly impossible to entirely enforce. This gives police the ability to selectively enforce said laws simply because they can't totally enforce it. As you noted this can be used to punish the poor far more than others. The combination of all of these factors and the ability to track these "offenders" down more easily, it isn't a far shot from what the parent poster was saying by comparing it to a debtors' prison.