Proportional to income is "the same" under the equivalence of time and money. A fine is some % of your income which is some % of your working time. The fine as a penalty should roughly be equivalent to time spent in prison, so that is some fixed amount of time which automatically translates to some lost amount of salary. Going to prison being an alternative to paying a fine when you aren't solvent.

Otherwise it isn't a penalty and is just the price of being permitted to do a thing which might be out of reach for the poor. That's just fundamentally unfair to permit the rich to do things we consider immoral if they are just able to afford it.