"We charge less than we pay our upstream, but it's ok because we'll make it up in volume."
Seriously though, they're saying that most deployments use less than 18 hours per day, and if they just let them stay up 24/7 regardless of that, they're basically leaving money on the table. Heroku pays Amazon for all of what we're getting for free. Their free tier is a loss-leader.
If they don't spin down your app when it's idle, it absolutely costs them more than if they do. Regardless of how much (or even whether) they bill you for it.
Seriously though, they're saying that most deployments use less than 18 hours per day, and if they just let them stay up 24/7 regardless of that, they're basically leaving money on the table. Heroku pays Amazon for all of what we're getting for free. Their free tier is a loss-leader.
If they don't spin down your app when it's idle, it absolutely costs them more than if they do. Regardless of how much (or even whether) they bill you for it.