In NJ, the registration fee is based on the age and weight of the vehicle (studies have shown a correlation between age, weight and value of a vehicle). The thought being that if you can afford a more expensive car, you can pay a higher registration fee. A quick Google search yielded a table that shows all states registration fee calculation bases. They all seem to have some sort of sliding scale based on factors that roughly correlate with a vehicle's value (weight, age, use, etc).
Looks like you whiners are out of excuses, because your hypothetical Dr. in the burbs IS paying a higher registration fee.