Being expected to tip a person for a service you just paid for is NOT normal!
Yes, in the EXCEPTIONAL situation where you want to give a person money for whatever reason, please do so but do not change a default setting for something that warrants it 10% of the time.
How does one justify going to a salon, being given a price by the barber, paying it and still be expected to "tip" the person you just paid what was asked for?
Uber including tipping will mess up the experience. Period. It will influence ratings. Cos, how would a driver rate 2 passengers with equal service but one tipped $20 and the other $0.
We from the rest of the world are very uncomfortable being forced to perform this "optional" task.