for-profit businesses do not have a generalized right to make their mistakes into others' obligations.

The OP never argued that he has the right to force the cost of his mistake onto Heroku. He's polling a community of business owners to see how they feel it should've been handled.

If I were Heroku management, I would've written the OP something like this:

(Valued Customer)-

I'm so sorry your accident created a nasty end of the month surprise. We understand, but we cannot forgive the whole amount because while they were not servicing requests, we incurred real costs keeping the dynos ready, willing, and able for you.

Since it's your first mistake of this sort, and you've been such a great customer, we're willing to reduce your total amount owed to $x.

We hope this helps, Heroku

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so yes, I think it's his own damned fault. But I also think it's a missed opportunity to create a happier customer.