We won't be charging people extra or forcing them to change their plan if you have a sudden influx of visits from Hacker News or another site. We will consider that a temporary traffic spike and we will be happy that you are in the spotlight rather than trying to charge you more! The plans are based on your usual monthly usage, so if you continued to get the same number of requests monthly, we would talk to you at that point about upgrading.
In other words, something akin to 95th percentile billing? Odd that you didn't use terms like that in the response. Most experienced hosts or ISPs would.