I get what you're saying... but the price they're charging is the price they're charging. The whole point is that I don't have to care how they make it work, just that it does work.

Would the price for Heroku magically become "better" if they decided to switch from Amazon to a more expensive or less efficient backend? That can't be right.

Heroku has many alternatives -- hosting directly on Amazon among them -- but (IMHO) there are not many direct competitors. Doing it any other way involves different tradeoffs of time, money, skill, and maybe lock-in risk. I sometimes buy a burger at a restaurant even though I know I can make one at home. That doesn't automatically mean it was a bad call even if raw beef is on sale at the supermarket.