Does Apple allow buying a new phone through its website the day it's released?
The only thing you're missing is the need to be part of the party.
People like queuing up to be first.

If you pre-order it online* you can answer the door in your pyjamas &amp; get your phone the same day, without the queue. I've known them arrive at 8am on release day.  

*Note: If you wait too long to pre-order, you won't get it on the first day, but some time later, depending on how popular the release is.

Announcements are usually made in the first week or so of September, with the release being about a week later. The online stores will go live for pre-orders at about the time the announcement/conference ends. They will be closed for the day up to that point.
No date has yet been set for even the announcement, so at present everything is still speculation.