Possibly they give you a small credit for the trouble, and maybe ding the previous rider somehow.
A car with a spill in it, bodily or otherwise, is probably out of commission for the night. Depending on the type of stain and time before treatment, it could take the car out of the fleet for a few days. Eventually self-driving cars have bus seats? idk.
No problem, we can make the rider accountable. We have cameras in the car. First issue here is you can't trust users alone to provide accountability. Say I put my leftovers on the floor. You get in next and don't notice. The next rider however complains of a strong fish smell in the car. The car is taken out of service and your reputation is dinged since you were the last person in the car.
That's no good, so we'll need humans to review video and find culprits. Spinning up such a team will require software, managers, and oversight. Being alone with your date in a self-driving car is _exciting_ so you definitely don't want your employees to freely copy or share private videos of riders.
Like I said, this stuff is all solvable with centralized teams, but it cuts into the profit fantasy a bit. And it simply didn't exist when every car in your fleet came with a chaperone incentivized to take care of your/their car.
These are all solved problems.
Also, you think those engineers, capable of creating an autonomous car, somehow will find it too difficult to install liquid detectors all over the car and combine them with cameras inside to would allow for instantaneous, remote inspection of the car and imposing the penalties/taking the car offline – all of which are possible with existing technology?