I suppose that one angle is not that one person is extracting a very small amount, it is that if you allow it then everyone can do the same.
When the punishment far exceeds the loss caused by the "crime" then it is absolutely unfair, it undermines the rule of law itself.
Just being arrested over it could be considered punishment itself. Especially if it's a young person who gets into trouble, it is traumatic for them. It is also sending the message that the system itself is unjust, and he/she might not think twice before committing a real crime, e.g. real theft or fraud when he/she grows up.
What about punishment as a deterrent? People can commit a crime many times and only get caught once. Should they be punished only up to the cost of the one crime they were caught doing?
Take, for instance, knives. In some jurisdictions and places, IIUC, knives above a certain size are illegal to possess in a public place (unless properly packaged and in the process of being transported). The cost of someone simply possessing a knife is obviously zero, but enforcement is expensive. So why is it (putatively) enforced? Because people possessing knives can commit other crimes, such as attacks and robberies. That is, one crime is enforced not because breaking that law is itself a cost, but because it might lead to other crimes, which are costly.