What happens when someone walks into an Apple store and they are sold out of the particular band/watch combo? "Sorry we have the watch, but not the band you want... sucks."
But that can happen when the watch/bands are packaged together, too.
So Apple has 18 steel watch/band combos on their website. They start out by having all 18 in stock. One of the 18 turns out to be very popular, and sells out in an hour. Now, customers have a choice of buying a watch with a band they don't want, or they can walk away without buying one.
How is that an improvement over, "sorry, we have the watch but not the band you want?"