Are there any purchase conditions that prevent unlocking phones designated as “iPhone 6, AT&T Compatible”. As long as AT&T does not force restrictions on unlocking there should not be any reason that such a phone could not be unlocked.
Once it’s in your hands, it’s between you and your carrier.
Does AT&T need to approve the unlock? (Would want the phone to operate on T-Mobile)
Were these made for the United States?
I bought a used one before but it was made for somewhere in South America and I had all sorts of trouble with it-unable to open/receive text texts-none with pictures at all. Unable to access the internet.
All it was good for was as a phone & camera.
I spent over an hour with AT&T tech support before we finally figured it out. So I am back to the old flip phone. 
Yes, they should be. If you have any issues, be sure to get with our customer service.