To me it kind of makes sense. Heroku is in effect taking the place of a number of developers and/or sysadmins, they've already thought about the problems and solved them in one particular way.

Sure they could re-write things and move to their own servers or amazon s3 or google computer or wherever but they'd have to go through the process of finding, training and hiring people to do it, plus all the difficulties that come with that.

I would imagine they're thinking about changing many things about their setup but for something of their size I can see why the money tradeoff makes more sense at the current time