I never thought of that- they don't just have aws availability to worry about, they have the availability of 79 external service providers, totally out of their hands, to worry about.
it wouldn't matter as much. if your primary app and workers via Heroku is down (which presumably are the ones using the add-ons) none of the end users are going to notice the add-ons being down.
On the other hand, Heroku could do everything in its power to keep its main system up, but the failure of a highly used addon, totally out of their control, could cause nearly as much of a problem for them.