Answer:
be “expected” to pay more. A person works really hard to get “wealthy”. They work hard so that life becomes easier on the long run that includes being able to afford stuff more easily. If the start paying for everything based on how much they earn there is no point in them working so hard. The biggest incentive of working hard is lost then. And productive of people come down when there is no incentive.
I understand your question was specific to fines but then later someone will say the wealthy should pay more for limited goods cause it's fairer that way, someone else will say the same about real estate and so on. So saying yes to fines based on income starts an endless chain which finally will bring down productive of the society on a whole