Earlier quoted context omitted.
Why? If I'm a poor speeding Fin, then a non-scaled speeding ticket could cost me a good number of days food. If I'm a rich speeding Fin then a non-scaled ticket is just the cost to travel quickly. When you're wealthy enough fines don't matter there's no point in the police enforcing them aside from as an additional income stream.
Speeding has a constant negative effect on society. It doesn't vary based on how wealthy the speeder is. Therefore, if the rate is higher for some people, it's either insufficiently disincentivizing poorer people from speeding, or excessively disincentivizing richer people. Pigovian taxes should be constant. And if it's not pigovian, why give tickets at all?
A flat fee doesn't discourage those with more money to pay it. It simply says "speeding is OK if you have enough money."