Interesting point, fine for speeding, the main purpose of the fine itself is to punish for breaking the law. If so, basing such punishment on income of the criminal makes more sense then set fee.
ie: you make 100$ month, you jaywalk - 10$ fee. There goes your money for fun for whole month, you WILL remember.
you make 10000$ a month, you jaywalk - 10$ fee. You barely notice, who cares if you get caught again ?