It may be an AWS outage as the cause, but ultimately it's Heroku's problem. They're the ones touting:

  "Erosion-resistant architecture.

   Heroku takes full responsibility for your app's health,
   keeping it up and running through thick and thin..."

The thick happened today, something eroded, and people are holding them to their word: full responsibility for their app's health. Heroku could do load balancing between multiple independent providers rather than be solely dependent on [one region of?] AWS.