Yeah, I'm sure some of the experience quality is because the only other riders are people Waymo selected for the beta, and nobody wants to get kicked out for misbehavior. We've joked a few times "don't forget that water bottle, I don't wanna get banned!"
On the other hand though, I have seen misplaced items in Waymos a couple times and when I reported them in the app they pulled the car for cleaning ASAP. I actually did leave a water bottle once - a disposable, single use one - and they emailed me asking me if I wanted them to return it! That kind of vehicle management feels more like a benefit inherent to centralized taxi fleets than a benefit inherent to AVs, but it's something Waymo can take advantage of. I would guess the in-car cameras would be capable of detecting vomit-related events etc too.