Scaling all penalties and fines to income sends the same signal to everyone. This system is not only fair, it makes economic sense given that the purpose of a system of fines and penalties are socially accepted deterrents for reducing socially unacceptable behavior.
If the size of the fine = price and the incidence of the socially unacceptable activity = quantity demanded, then using a sliding scale for the fine takes into account the inelasticity of demand for those who are wealthy.
Most of Scandinavia determines fines based on income. Could such a system work in the U.S.?