nope, those websites would be just as lazy or stupid for shutting their site down, there are tons of ways to develop new updates and features that do not affect the current production environment and allow you to incrementally roll it out when it's ready. I'm not sure if you know them or work there or something, but if you give me a specific example of when the site was shut down that you thought was reasonable, I am happy to explain why it was not and how it could have been avoided.
the only case I can think of where it might legitimately make sense to disable a website entirely during an upgrade, is when there is somehow life and death involved, although even those websites would be tiered into a multi-stage extremely slow and cautious roll out, so there wouldn't really be a need for a particular site to go down during the rollout that I can think of