You're going to see a lot of "Apple chat rep told me it's locked" or "Apple store says they'll be locked" ... remember it's the activation profile that tells the phone's lock state. So when you buy it, if you buy it at a discount, it's locked to ATT through a provisioning profile. If you buy it without a contract, there's no lock. It's been that way for several years now.
I remember when I bought a T-Mobile (unlocked, lol) at full price last year, I told them I was using it for AT&T and the guy in the store said no it's locked to T-Mobile. I explained to him that I'd bet him lunch that when he rang it up at full price and I put my sim in, it's going to say "Congratulations your Phone Has Been Unlocked" ... he took the bet. I turned out right. I didn't make him buy me lunch, but we got a good laugh out of it, probably less funny for him tho.
I remember when I bought a T-Mobile (unlocked, lol) at full price last year, I told them I was using it for AT&T and the guy in the store said no it's locked to T-Mobile. I explained to him that I'd bet him lunch that when he rang it up at full price and I put my sim in, it's going to say "Congratulations your Phone Has Been Unlocked" ... he took the bet. I turned out right. I didn't make him buy me lunch, but we got a good laugh out of it, probably less funny for him tho.