Earlier quoted context omitted.
The way Uber works now is it incentivizes the driver to give you a poor rating if you don't tip. By including the tip functionality in the app, and preventing the driver from seeing the tip before you are rated, it will remove this incentive. I don't like tipping either but Uber had to do this and I'm confident the data showed it.
I used to tip in cash to avoid the bad rating especially if I am travelling to airport and have heavy luggage. I hate carrying cash and this is a very welcoming change. Anything that would make them look less evil(Although i don't think they are any worse than most companies.) is a good PR move.
Perhaps you live in a market where Uber drivers are aggressive about asking for tips, though, which is not the case in SF.