I didn't realize you could tip with Uber. They certainly don't make it easy like Starbucks does.

only with cash

How very un-Silicon.

Compulsory tipping allows employers to keep wages low forcing employees to rely on tips to make a living. How often do you see/hear in a job posting "make great tips" this is just code for we are not going to pay you a living wage? Tipping has its place but it shouldn't be required. I'm tired of supplementing the business' costs.
A good friend said once "if you can't afford the tip, don't got out to eat". That's another viewpoint I consider also.