Yea they don't really sell Unlocked iPhone devices directly on the Costco Website. There is just partner links for AT&T, and T-Mobile and you can qualify for a $200 Costco Gift card. Start here for more information:
#2 is probably right. Whenever anything seems less customer-focused, think about the money. Is making a change that seems even anti-consumer more profitable? If you can see the way to a "yes" answer, you probably have figured out the "why?"