I think that's true with Heroku, but gets less true as providers get bigger. If you tell a user that your service is temporarily down "because Google is down", even a lot of regular people will know what that means, and not really blame you for it.
Not necessarily true, especially if your customers are management types looking to lay on the blame as thick as possible wherever they find it, and will terminate a relationship if they believe the service-provider is incompetent.
For those kinds of customers, they may understand what Heroku is and why their vendor is using it and will definitely make at least some distinction about outage fault.
