I totally agree with you (the first part). It's all calculated risk, and evaluation of cost.. you keep it up when the math works out. If it's down and it hurts your customers to the point that they'll walk.. it's on you. Bad math, your problem.

I speak as someone who's worked primarily in healthcare building services where I assure you we held ourselves personally responsible for natural disasters.

So it depends on your app. If my startup lets people take photos of their dessert and paste lolcats on them, then maybe my hosting goes down and I show my users a page that says the server must have farted, who cares.. but the last thing I'd do is show a page that said the people I pay with their money must be fucking up at the moment and we'll all wait together for things to get better.

Point is: My users are not my peers, they're my responsibility and livelihood. Even when something totally out of my control occurs. Fuck, especially when something out of my control occurs.

It is their fault. The company could have used global load balancing with the app hosted in multiple geographically distributed data centers in multiple national jurisdictions through multiple independent providers. This would ensure that the earthquake which leveled Amazon's California data center and the flood which took out Hetzner's data center in Germany and the martial law declaration which took out Linode's Japanese data center and the bankruptcy which closed down Rackspace's Amsterdam data center has no operational impact on Peer1's data center in New York where the service continues uninterrupted. As the data is fully synchronized between all data centers, the company keeps running, all customers are online, and the company can work on setting up additional redundancy in a Canadian data center to make up for the others which were lost.

Yes, this costs money. It's why people accustomed to getting everything for free on the internet can't fathom why larger companies charge six or seven figures for a service that they could roll out themselves by installing an open source package on some Linode VM. If you're paying that kind of money for the reliability, it's because you're extending a promise to your end customers, and the service contract you receive from your provider should come with lots of guarantees and financial penalties if the conditions warranting the price tag aren't met.

It is absolutely your fault if the site does down due to natural disaster. This is why companies who are large enough to do so host the site at multiple locations with the ability to failover.

Customers usually don't care about the reason for outage. They gave you money. If the service is running, good. If it's not, you screwed up. No matter what actually happened, you should've been prepared. Sad but true.