I wouldn't phrase it as "choosing not to sell them". Apple is going to sell every single watch they have for the next few weeks (or months). The demand is greater than the supply.
And there are a number of logical reasons to have a daily quota:
- They want to keep the stores relatively calm and orderly (think of what black friday is like for retailers). Apple would rather keep a steady flow every day for a few weeks than have a stampede the first day.
- The store has business hours. Would you rather tell all of the customers that have been waiting in line for an hour that "sorry you waited, but we're closed. try getting here earlier tomorrow" or tell people throughout the day "sorry, we sold out for today, please try again tomorrow". The end result is the same "come back tomorrow" but I'd rather be told that WITHOUT waiting line.
- They know that they can only sell N number of watches per day (N = number of employees * number of watches a single employee can sell per hour * number of business hours in a day). So don't let more than N people reserve a watch to pick up in a day.
- You have reservations spread throughout the entire day. If you have too many walk-ins, that will delay the people with a reservation. They'll be angry for two reasons: 1) What's the point of a reservation if people can just walk in off the street and get service sooner 2) Their reservation is delayed by all the walk-in traffic.
That's just a few I can think of.