If anything, jailing a low income worker means they loose their job and have trouble finding another one after getting out while jailing a business owner or stock trader etc. will mostly mean they are in the same position when they get out as when they were jailed.
Similar, even fines proportional to income are still unfair because to determine how much a fine hurts you need to compare it to whats left of the income after basic needs have been paid for. For someone that has much more than they need, getting fined 50% of their income will suck but not immediately change their lifestyle while someone who is living paycheck to paycheck is going to be ruined by the same percentage fine.
If you jail different people, they lose out on different amounts of income. Is that unfair?
Now remove the physical jail component and keep the rest of the punishment. Is that unfair?