> I'm sure there are gray areas in such contracts but something being down or not is pretty black and white.

Is it? Say you've got some big geographically distributed service doing some billions of requests per day with a background error rate of 0.0001%, what's your threshold for saying whether the service is up or down? Your error rate might go to 0.0002% because a particular customer has an issue so that customer would say it's down for them, but for all your other customers it would be working as normal.