It was not sanctioned by them, but they're still responsible for the actions of their employee.

What if it was a customer trolling a restaurant they didn’t like?

Do you routinely interview, hire, and give responsibility to your customers?

Then the employer doesn't compensate in this case.

I mean, Chicago pays something like 100 million dollars a year in police misconduct settlements...

You mean such as in the case of e.g. police brutality or mishandled prosecution? The government is indeed on the hook. It is OpenTable's responsibility to ensure that it hires ethical people and that it encourages them to behave ethically and verifies they are doing so. Anything else would be insanity.

Yes, the government pays out settlements due to employee misconduct too.