I am baffled that the return was accepted.
Why would you be baffled that Apple honored their return policy?
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Apple's retail store policy (and online, too) says they can refuse sale if they suspect you are buying with the intent to resell. My guess is that it is rarely enforced for small sales.
Also, the Verizon one is unlocked internationally. It says so on Apple's website. The old version wasn't, which might have confused the sales person. The AT&T version is the same one sold internationally. However, the Verizon one has exactly the same 3G coverage as the AT&T one. I'm not sure which version Canada gets. But outside North America the AT&T and Verizon one work identically.
Unfortunately, they've managed to make the Verizon model unlocked for any GSM carrier EXCEPT AT&T. Unbelievable.