I think it's a combination of money laundering and phone scams where people are told they owe money to the IRS or something and are tricked into buying a bunch of gift cards.

That said, if buying and redeeming gift cards are such an indicator of fraud that people are legitimately afraid of getting their accounts permanently locked, why doesn't Apple just stop selling them?

Apple keeps money from the gift card after banning you. Just business, nothing personal.