I do not think receiving tip thru bitcoin breaks any rule. Tips are given out of the generosity of the passenger for the good experience or service. It is besides the fare. If there are a lot of bitcoin users in their community he might get a tip. Otherwise, it is a good way to spread awareness of bitcoin.
I cant conclude regarding with this because its on Uber management if they allow this kind of thing regarding on bitcoin tipping but as you said recieving tip would not break any rules but for sure uber would get a percentage of those tips if they are aware with it.