This seems to be saying that Uber drivers expect a cash tip in the US. If not received, they'll give you a low rating?
I always tip via the app when in the US. It may be hours later (at the same time I give my rating), but I always do it.
If i'm getting a low rating, even after tipping, then I'll stop tipping all together when in the US. Note - I rarely carry cash (who needs to these days?).