It does punish everyone equally, if everyone pays the same fine. Some people having more ability to pay does not make the law unjust.

I think it's important to remember that money represents debt. When someone commits a crime, they owe a debt to society. But if they have money, that means society owes a debt to them, so when they pay the fine it balances out.

The system isn't perfect but the idea is that if someone makes a big contribution to society, like by practicing medicine or creating new technology, society's debt to that person shouldn't be cancelled out by a minor offense like a parking violation. But if they aren't contributing much, then breaking the rules could make them into a net negative.