In my opinion, Ghost made a mistake in terminating the service without prior notice (although I think it would be okay for them to do so with prior notice). The terms of the service allow doing so for any reason, but that's no justification. In fact, having such a term is a good reason to avoid Ghost.
Also, I find it interesting that Ghost didn't answer "What happens if an article goes viral and it breaks the 100k views boundary? Automatic charge?" question. The pricing page provides a reasonable response here.
> We never disable sites for traffic spikes, so no need to worry about the front page of Reddit or HN. If you exceed your limit consistently on a 3 month rolling average, we'll just let you know that you need to upgrade.