There is a difference in one aspect. Generally, Amazon sells single products. When they add a new product, they just add that one product. It doesn't affect anything else on their site, for the most part.
Apple, on the other hand, sells many integrated products. When they add a new display, they may need to update the pages for each and every product that deals with that display. When they announce new software, they must update the build-to-order pages to include that software. But they can't update the pages while the store is open and someone may be actively buying a new system. In that case, they would be changing pages that people are currently using...that would cause some issues. Yes, technically, a page is loaded then changes don't affect it, but what if you reload, or go back and come back to that page, etc?
I think this may be why they close the store while updating whereas Amazon does not. Make sense?