I'm not sure this is correct. Activation doesn't lock them to a carrier.
Activation per se doesn't lock the phone, but when you are paying for the phone with a payment method that requires carrier lock, then the phone is locked during the activation process. So if you are buying a phone using one of the carrier upgrades or payment plans, you can't leave the store without activating the phone. At least, that's how I think it works, but if anyone knows different, please feel free to correct me.
It's just that I've never gotten the feeling that at Apple Stores, there are separate stocks of carrier locked and unlocked phones. They don't always ask you how you are paying before bringing out a phone -- they ask what model you want, bring it out, then it's "how are you paying for this?"