Two iPhones Per Person
This has nothing to do with insuring availability of iPhones. It's about shutting down the gray market in iPhones, particularly unlocked iPhones.
The availability of various software unlock schemes has led to a vibrant gray market where people buy as many iPhones as they can, unlock them, and sell them, mostly to the international market, for a very nice profit.
This is part of the ongoing cat and mouse game between Apple and hackers and hustlers. I think in the long run Apple will lose this game.