From a retailer perspective, it is much easier having people come in at their liesure to purchase shoes after they have won the RSVP.
Before people would have to line up for hours/days on end, security would ned to be arranged for some releases, then you have to deal with people cutting or trying to barge inside the shop. Not really worth it. Nike was essentially forced into this setup. Though Twitter won't be around forever, so Nike will need to implement a new way for people to win the chance to buy them eventually.
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