Heroku is generally regarded as a reliable hosting solution. If it was a bargain basement alternative that was expected to fail randomly, then this would in fact be "clownshoes" downtime.

Building in full redundancy behind Heroku is possible, but non-trivial. They are after all being paid to provide a reasonably fault tolerant solution.

Therefore, while not literally unavoidable, it may not make good business sense for a venture to incur the additional expenses associated with implementing full redundancy. Most of their clients probably understand that downtime happens and will be perfectly content as long as every reasonable effort was made to keep the system online.

edited to add : Communication is key. Business relationships typically don't crumble due to aberrations like this (power outage, hosting provider going down, etc.). However, they do crumble if communication either doesn't occur, or the wrong information is conveyed. That is the crux of this discussion.