There is one side of the political spectrum that feels that the penalty for a crime should be set irrespective of the perpetrator because that's fair. Two people that commit the same infraction pay the same absolute amount.

There is another side that feels the penalty should "hurt" the same amount because that's fair. Two people that commit the same infraction feel the same amount of pain (theoretically), roughly corresponding to paying the same relative amount.

IMO this falls apart when you accept the almost tautological fact that these laws are enforced selectively, so "fairness" goes out the window almost immediately. Enforcement is used as political pressure and as punishment. Under that view, the second option above feels much worse than the first.

Even if enforcement was "unfair" (let's ignore for a fact that this is not a binary determination and not being able to be perfect isn't an argument for not trying) then everyone having the potential to experience the same hurt from the unfair system is still more fair than a corrupt society where some people can have their lives destroyed by an unfair fine but others can just shrug it off.