That article is somewhat confusing. The issue isn't that Uber doesn't allow tips, it's that when a person does decide to tip , Uber takes 50% of it.

Here is a much better writeup: http://www.theverge.com/2013/8/28/4668504/uber-lawsuit-san-f...

I've used the app several times and I've never been prompted to enter a tip. Maybe it's a new feature?

Edit: After reading a few more sources I think the issue is that every fare has a built in 20% tip, but Uber takes half of it. Another source: http://www.businessinsider.com/uber-drivers-open-lawsuit-ove...