whenever there are a short of supply, there will be 'grey markets'. plus apple can't really control how many pairs one can buy?
I mean, you can limit purchases on one credit card, but people will just go buy those pre-paid Visa cards, or walk into a store with cash.
or, you can limit purchases on one account. then people will just use guest checkout, or create multiple accounts.
or, you can limit purchased shipped to one address. then how many should you limit? I may have a 'shared address' of 10 people, or maybe I have a big family, with multiple people living in one house, and each and every one of us want some extras as gifts.
so, there's pretty much no good way to control who buys it and how many one buys, without causing significant customer experience issue.