"Tipping isn't obligatory" -- yes, it kind of is. Uber used to bar drivers
    form asking, but they recently lost a lawsuit over that rule and so now Uber
    drivers will occasionally ask for tips (which will cause it to slowly become
    the norm).

I'm pretty sure I was recently given a low rating by a driver on Uber because I didn't tip. From my perspective the ride went perfectly fine, and I wasn't even asked for a tip, but I checked my rating later and it went down from the perfect 5.0 I used to have.

If this becomes the norm, tipping will become mandatory because all the drivers will know who tips and who doesn't and will adjust service (or lack of service) accordingly.