First of all, there is no indication that there was only one $20 fine, the article just stated that fines are "perhaps $20" and that the court costs can be up to $150. That is no indication of the situation in this case. So your conjecture is not warranted.
Only the government can do this.
The last I checked, credit card companies and other creditors can assess exorbitant late fees, over-limit fees and the like. So, no, it's not "only the government" that does this.
Yes, it is.
MasterCard cannot decide they don't like what I am doing and tell me I now owe them money.
If they do I am free to laugh in their face.
Even if I enter into a contract with MasterCard and am unable to repay money they lent me all I have to do is declare I am bankrupt. At that point their only way of punishment is to decide not to ever lend to me again.
But the local government, whom I never entered into any contract with, can decide that I am not doing something they want to force me to do and so they fine me and put me in jail when I can't pay the fine.