Feinberg observes that in 1973 when he wrote his article, the top 5% of households in the USA gain 20% of the income in the USA while the bottom 20% of households gain 5% of the income. (In 2011, the top 5% of households in the USA gained 22.3% of the income in the USA while the bottom 20% of households gained 3.8% of the income.) Why does Feinberg think that this is unfair?