Here's looking at the history of what it used too say on Uber's Help section of the app:
Before tipping was available:
"Do I need to tip my driver? You don't need cash when you ride with Uber. Once you arrive at your destination, your fare is automatically charged to your credit card on file - there's no need to tip."
When they released the tipping feature:
The language has softened but they still weren't so encouraging. It says "In most cities, Uber is a cashless experience. Tipping is voluntary. Tips are not included in the fare, nor are they expected or required. As a rider you are not obligated to offer your driver a gratuity in cash. If you decide you would like to tip, your driver is welcome to accept."
Now, this is what it says. I believe this happened in mid-2017:
The language is even better now. "you are free to tip and drivers are free to accept. You can tip your driver at the end of your trip through the app. "