Uber's claim is that black people are known to tip poorly, and so drivers would be incentivized to cancel their rides in favor of fares they expect will tip better. Therefore they don't allow tipping to prevent racism. Supposedly...
Drivers are now allowed to ask for tips; I've had drivers with tip jars and squares available. And I've seen reddit discussions of drivers calling riders who don't tip dicks.
https://hbr.org/2016/05/ubers-new-tipping-policy-is-a-mistak...
Suggesting that black people are somehow to blame for Uber not allowing in-app tips is one of the most out there things I've seen on HN, and that's saying something.
http://www.esquire.com/food-drink/restaurants/a23132/why-tip...
When they were sued by their own workers because of this and had their backs against the wall, they had to claim it was because of this racist hand-wavy BS.
It was a ret-con and an incredibly cynical and insulting thing to do, but that's now the official reason they didn't allow tips.
https://www.bostonglobe.com/business/2016/04/27/uber-resists...