Nowhere do they "clearly say" tips are welcome. They add on to any language that might be construed to welcome tips that they are "not necessary, not required, the fare is expected to fully compensate the driver" etc. ad nauseum. People are also misled into thinking tips are included or added in after the fact, due to their previous actions and wording in the app and the ubertaxi tipping option, which many don't even understand is a separate service from uberx. Everything they do and say is designed to dissuade people from tipping and/or fool them into thinking they already did.