Yeah been wondering this myself. I think that's how Heroku really emerged in the scene, AWS gave you the ability to scale but didn't make it easy by any means (relative to 'heroku scale web n'). I doubt billing was the issue either considering tools for billing monitoring and email warnings have been in place for a while. My guess is that things that don't scale easily (db, workers, etc) which, are what most EC2 is used for, have to be scaled with an API and consequently that unprioritized webserver type autoscaling.