> How is it fair? I would think if two parties commit the same crime they should be charged the same. Isn't fairness in law defined as being blind to the perpetrator?

The purpose of a fine is not supposed to be a fee for a crime but a penalty that has deterrent effects. A flat fine is not an equal deterrent for people of different financial means.

In Finland the system is called "day fine", meaning it should match approximately a day of labor/income. In some situations you can even go to sit in prison for time proportional to the day fines, although this is nowadays rare.