Everything about modern pricing theory -- from menu design to coupons to sales to promo codes advertised only to specific markets -- is about getting people who can pay more, to pay more, while still making a little bit of money on those can only afford the bare minimum.

A millenium ago when you bought everything haggling in the market, the rich guy was paying 3x as much as everyone else because the vendors knew they could make him. Meanwhile, the impoverished grandmother gets the unsold leftovers for free. (The specifics obviously are made-up, but you get the gist. It's still the same in much of the world today. Foreigners visiting China pay much, much much more than locals.)

This is the same human principle, just data-driven. It's kind of like progressive taxation, too: those who can afford to pay more, do.

Now you may object: Uber doesn't need the money. But it does need to be able to sustain itself long-term. So much like first-class airplane fares allow economy flights to be as cheap as they are, similarly Uber passengers with the ability to pay more are exactly what make it cheap enough for other passengers to afford at all.