My guess is they're ramping up instances/servers to increase the scaling needed to handle the first-day traffic, which means they need to push/clone their code/assets that may take a bit of time. It is best to do this when there are no open connections to the store/database, which would reduce the time needed to push changes to all instances. After the traffic dies down, there is no point to have extra servers running, it costs money and it is energy-inefficient, which is why they shut them down and repeat the process each time after each major product comes on the store.
We're not talking about a few thousands of users rushing the site, we're talking millions of folks at the same time, each person has to log in, authenticate, add/modify/confirm credit card information, etc, and so on.
Also, they're only doing this for the store, the other parts of the website stays up just fine, so they're not turning off the website completely.
