If you are a company who is using Heroku, you likely do not have the resources (knowledge or money) to set up a system which degrades gracefully when your entire hosting platform goes down. That's a hard, and expensive, contingency to plan for.

And even if they did build their service in a way which could tolerate a complete hosting platform failure - degrading a core service during your customer's business hours is enough to make heads roll. It's just stupid.