You don't understand your own analogy.
The Avengers opens May 1, but there will be incredible demand and the chances are good you will have to stand in long lines at the theater until there is an opening for you to buy a ticket. Most shows will sell out until the demand subsides, so you might not even get in the first day. You can always pre-order online quickly when tickets are put on sale to ensure that you get a seat, but shows can sell out online as well.
How is this scenario any different than the Apple Watch? In both cases, the product arrives on a certain date, but pre-orders can sell it out before that date arrives, so there is nothing for a walk-in on launch day to purchase.