It's all about economics. When the risk of buying used iPhones outweigh the cost benefit, people will be less inclined to buy used iPhones. As the upgrade pressure accumulates, consumers will be lured to purchase new iPhones. Since Apple does not profit from 3rd party used phone market transactions, and the longer an iPhone's life cycle is, the more it cost Apple to support and maintain, it is in Apple's interest to control all transactions. When used iPhones are sold back to Apple at a lower-than-market price, Apple can then sell refurbished outdated iPhones to emerging markets. Capitalizing on those used iPhones is even more important when primary market is shrinking.