If you know that the alternative costs twice as much, and you doubt that the driver offering a ride for cheap can actually support his family, you have a choice.
If you are worried someone else might take a chunk of the expensive ride, or if you prefer the cheap ride anyway, you could always tip. US is the land of tipping, right? But then again, that might feel weird since the payment is centralized... Funny, that.
I do agree that we don't have a moral imperative of choosing the cheapest option, but who said we did?
I also don't agree with your language, by the way. Uber's drivers are not using Uber; Uber are using them.
I meant the drivers are using Uber, the app. Whether the drivers are being "used" is a consideration I'm happy to leave to someone else.