Two people make $10 per hour.
Person A works 50 hours a week and thus earns $500 a week.
Person B works 20 hours a week and earns $200 a week.
Person A and B get a ticket. Because Person A makes twice as much, he is fined $100. Person B is only fined $40.
Alright lets compare it to the status quo:
Three people, A, B, C, make $10, $200 and $200 per hour.
Person A works 50 hours a week, earning $500 per week.
Person B works 20 hours a week, earning $4,000 per week.
Person C works 50 hours a week, earning $10,000 per week.
They all commit a crime which has a punishment of $500 (or with an earnings linked system, max{500,1weeks income}.
Current system: A gets punished $500, and is deterred from committing the crime since it's a weeks income.
B gets punished $500 and it is of little deterrent seeing as it's only 12.5% of their weekly income.
C gets punished $500, and it's of almost no deterrent, being just 5% of their weekly income.
Alternative system: A gets punished $500 and is deterred.
B gets punished $4000 and is also deterred.
C gets punished $10k and is also deterred.
So the principle aim of the punishment, to deter people from doing it, is met. Now lets look at the less important side, the 'equity' issue:
Current system:
A is punished 1 weeks income, or 50 hours of labour
B is punished 12.5% weekly income, or 2.5 hours of labour
C is punished 5% of weekly income, or 2.5 hours of labour
Alternative system:
A is punished 1 weeks income, or 50 hours of labour
B is punished 1 weeks income, or 20 hours of labour
C is punished 1 weeks income, or 50 hours of labour.
It's not perfect - B is still being punished in fewer 'hours of 'labour' than A (or now C), but the difference is greatly reduced under the new system. Furthermore it is actually a deterrent in the new system, while it isn't in the old system. To focus on the problems with the new system while ignoring the even greater issues with the current system isn't consistent. Now maybe you might want to improve on it by having the fine look at someones hourly income, and take into account wealth, but you also have to bear in mind the practicalities of that, which might make such an approach too complicated and not worth it. That doesn't mean that the deterrent and equity benefits of the new system should be ignored.