I'd certainly be happier paying more per pro-rated unit of time that I actually use than I am feeling as though I'm in an arms race to not be fleeced.
Is there really that much revenue to skim off the top? Either way, they're paying Amazon out of those charges.
But I'm not so cynical as to think that their success is reliant on this income.
I'd be extremely surprised if Heroku actually spools up EC2 instances the moment you move the slider. It'd be a far more intelligent (and profitable) design to simply make sure there are sufficient resources to maintain the desired service levels with a buffer to deal with spikes in demand.
It should be relatively straightforward to provide service levels that are functionally identical to actually starting each "dyno" or "worker" with a greatly reduced cost structure.
Relying on customers overspending due to technical deficiency seems like a pretty poor business model.
I didn't mean to say they're relying on it, simply that it's adding to the bottom line and it would be greatly missed.